Electric power steering apparatus, control method thereof and computer readable medium

ABSTRACT

The electric power steering apparatus is provided with: a steering torque detector that detects steering torque of a steering wheel; an electric motor that applies steering assist force to the steering wheel; a current detector that detects an actual current actually supplied to the electric motor; a target current setting unit that sets a target current to be supplied to the electric motor, on the basis of the steering torque detected by the steering torque detector; a feedback controller that performs feedback control so that the target current and the actual current coincide with each other; and a feedforward controller that performs feedforward control for increasing the actual current detected by the current detector if the target current increases, the feedforward controller including a frequency compensator that provides a smaller amount of increase in the actual current as a frequency of a variation in the target current is lower.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC §119 fromJapanese Patent Applications No. 2009-039754 filed Feb. 23, 2009, No.2009-045285 filed Feb. 27, 2009, and No. 2009-085705 filed Mar. 31,2009.

BACKGROUND

1. Technical Field

The present invention relates to an electric power steering apparatus, acontrol method thereof and a computer readable medium storing a program.

2. Related Art

Recently, an electric power steering apparatus has been proposed, whichincludes an electric motor in a vehicle steering system and assists adriver with his or her steering force by use of power of the electricmotor.

The electric power steering apparatus is controlled by a controller. Tocontrol the electric motor drive, the controller first sets a targetcurrent to be supplied to the electric motor in accordance with steeringtorque, vehicle speed, and the like. Then, the controller performsfeedback control so that a deviation between the target current and anactual current would become zero, in order that the actual currentactually passing through the electric motor may coincide with the settarget current.

For example, an electric power steering apparatus disclosed in PatentLiterature 1 performs a proportional action of multiplying, byproportional gain Kp, the current deviation between the target currentand the actual current, and an integral action of multiplying, byintegral gain Ki, the integral value obtained by integrating the currentdeviation.

Under the feedback control alone, however, response to a change in thecurrent passing through the electric motor may not be necessarily saidto be sufficient. Thus, there has been proposed an approach offeedforward control execution in which a motor drive signal increases inmagnitude in accordance with the target current, in addition to thefeedback control, to thereby heighten steering response. (See PatentLiterature 2, for example.)

To control the drive of the electric motor, the controller, whichcontrols the electric power steering apparatus, determines the currentto be supplied to the electric motor on the basis of detected steeringtorque.

There has been proposed a technique in which, at the time ofdetermination of the current to be supplied to the electric motor, whichis based on the detected steering torque, a phase compensator is used toprovide a phase compensation of the detected steering torque in order toenhance the stability of the steering system, and the current issupplied to the electric motor in accordance with the phase-compensatedsteering torque. (See Patent Literature 3, for example.)

Patent Literature 4 also proposes the following technique. Specifically,with the fact taken into account that oscillation occurs at a resonancefrequency in a resonance system including a spring element that formsthe torque sensor and the inertia of a steering wheel, this techniqueprovides a band-elimination filter for eliminating resonance frequencycomponents of a signal from a motor drive control system on the outputside of a torque sensor.

Patent Literature 1: Japanese Patent Application Laid Open PublicationNo. 2001-315657

Patent Literature 2: Japanese Patent Application Laid Open PublicationNo. 10-100914

Patent Literature 3: Japanese Patent Application Laid

Open Publication No. 10-167086

Patent Literature 4: Japanese Patent Application Laid Open PublicationNo. 2-164665

Here, characteristics of a system that performs the feedback control onthe basis of the current deviation between the target current and theactual current are determined by characteristics of a transfer functionof the actual current relative to the target current. For example, thestability of the system is determined by a denominator of the transferfunction, and response to the actual current is determined by anumerator of the transfer function. Then, a system that performs thefeedback control using the proportional gain Kp and the integral gain Kiis incapable of effecting a change only in any one of the denominatorand numerator of the transfer function, regardless of how theproportional gain Kp and the integral gain Ki are changed. In otherwords, the system is incapable of setting the stability of the systemand the responsiveness to the actual current independently of eachother.

Therefore, an object of the present invention is to achieve animprovement in the responsiveness to the actual current withoutaffecting the stability of the system.

Also, the improvement in the responsiveness is effected by performingthe feedforward control in addition to the feedback control. However,even in a steady state, an adder that adds together a feedback controloutput and a feedforward control output has addition of the feedbackcontrol output and the feedforward control output, and thus, even in thesteady state, a difference arises between the target current and theactual current. Hence, the mere addition of the feedforward control tothe system that performs the existing feedback control might greatlyaffect stationary characteristics of the electric motor. Therefore, anovel system including the existing feedback control is required for theaddition of the feedforward control without affecting the stationarycharacteristics of the electric motor.

Also, the electric power steering apparatus drives the electric motor inaccordance with steering torque applied to the steering wheel, andtransfers drive power of the electric motor to a pinion shaft that formsa pinion-and-rack mechanism. Further, the drive power of the electricmotor is transferred to a rack shaft through the pinion shaft, andeffects a rectilinear motion of the rack shaft thereby to change thedirection of a wheel to be turned.

For suppression of vibration in the control system of the electric powersteering apparatus, it is therefore important that the electric motor,the pinion shaft and the rack shaft also be considered inertialelements.

SUMMARY

In order to attain the object, in the present invention, there isprovided an electric power steering apparatus including: a steeringtorque detector that detects steering torque of a steering wheel; anelectric motor that applies steering assist force to the steering wheel;a current detector that detects an actual current actually supplied tothe electric motor; a target current setting unit that sets a targetcurrent to be supplied to the electric motor, on the basis of thesteering torque detected by the steering torque detector; a feedbackcontroller that performs feedback control so that the target current setby the target current setting unit and the actual current detected bythe current detector coincide with each other; and a feedforwardcontroller that performs feedforward control for increasing the actualcurrent detected by the current detector if the target currentincreases, the feedforward controller including a frequency compensatorthat provides a smaller amount of increase in the actual current as afrequency of a variation in the target current is lower.

Here, it is preferable that the feedforward controller further include aweighting processor that increases or reduces an amount of increase inthe actual current by performing multiplication by a weighting factorset in advance.

Further, it is preferable that the feedforward controller furtherinclude a weighting factor setting unit that sets the weighting factorin accordance with at least any one of a vehicle speed of a vehiclemounting the electric power steering apparatus and an amount of changein the steering torque detected by the steering torque detector.

Furthermore, it is preferable that the weighting factor setting unit setthe weighting factor larger as the vehicle speed is lower.

Moreover, it is preferable that the weighting factor setting unit setthe weighting factor larger as the amount of change in the steeringtorque is smaller.

Here, it is preferable that the frequency compensator be a band-passfilter that transmits a predetermined frequency alone.

From another standpoint, in the present invention, there is provided acontrol method of an electric power steering apparatus including:detecting steering torque of a steering wheel; detecting an actualcurrent actually supplied to an electric motor that applies steeringassist force to the steering wheel; setting a target current to besupplied to the electric motor, on the basis of the detected steeringtorque; performing feedback control so that the target current and theactual current coincide with each other; performing feedforward controlfor increasing the actual current if the target current increases;providing a smaller amount of increase in the actual current as afrequency of a variation in the target current is lower.

From further standpoint, in the present invention, there is provided acomputer readable medium storing a program, the program comprising thesteps of: detecting steering torque of a steering wheel; detecting anactual current actually supplied to an electric motor that appliessteering assist force to the steering wheel; setting a target current tobe supplied to the electric motor, on the basis of the detected steeringtorque; performing feedback control so that the target current and theactual current coincide with each other; performing feedforward controlfor increasing the actual current if the target current increases; andproviding a smaller amount of increase in the actual current as afrequency of a variation in the target current is lower.

In order to attain the object, in the present invention, there isprovided an electric power steering apparatus including: a steeringtorque detector that detects steering torque of a steering wheel; anelectric motor that applies steering assist force to the steering wheel;a current detector that detects an actual current actually supplied tothe electric motor; a target current setting unit that sets a targetcurrent to be supplied to the electric motor, on the basis of thesteering torque detected by the steering torque detector; a proportionalcontroller that performs a proportional action of multiplying, by aproportional gain, a value corresponding to a current deviation betweenthe target current set by the target current setting unit and the actualcurrent detected by the current detector; an integral controller thatperforms an integral action of multiplying, by an integral gain, anintegral value obtained by integrating the value corresponding to thecurrent deviation; and an addition unit that adds together an outputvalue from the proportional controller and an output value from theintegral controller and outputs a command value to the electric motor.At least any one of the proportional controller and the integralcontroller includes a correction unit that performs multiplication by acorrection factor thereby to enhance an effect of a corresponding one ofthe proportional action and the integral action, and the electric powersteering apparatus further includes an adjusting unit that performs anadjustment so that a denominator of a transfer function remains constantregardless of a value of the correction factor, when the target currentset by the target current setting unit is an input, and the actualcurrent actually supplied to the electric motor is an output.

Here, it is preferable that the proportional controller include acorrection unit that performs multiplication by the correction factorthereby to enhance the effect of the proportional action, the adjustingunit multiply, by a factor depending on the correction factor, theactual current detected by the current detector, and output amultiplication result, and the addition unit further add an output fromthe adjusting unit, and outputs the command value to the electric motor.

In addition, it is preferable that the factor depending on thecorrection factor be a value obtained by multiplying, by theproportional gain, a value obtained by subtracting 1 from the correctionfactor.

Moreover, it is preferable that the integral controller include acorrection unit that performs multiplication by the correction factorthereby to enhance the effect of the integral action, the adjusting unitmultiply, by a factor depending on the correction factor, an integralvalue obtained by integrating a value of the actual current detected bythe current detector, and output the multiplication result, and theaddition unit further add an output from the adjusting unit, and outputsthe command value to the electric motor.

Further, it is preferable that the factor depending on the correctionfactor be a value obtained by multiplying, by the integral gain, a valueobtained by subtracting 1 from the correction factor.

Furthermore, it is preferable that the correction factor vary inaccordance with at least any one of a vehicle speed of a vehiclemounting the electric power steering apparatus and an amount of changein the steering torque detected by the steering torque detector.

From furthermore standpoint, in the present invention, there is providedan electric power steering apparatus including: a steering torquedetector that detects steering torque of a steering wheel; an electricmotor that applies steering assist force to the steering wheel; acurrent detector that detects an actual current actually supplied to theelectric motor; a target current setting unit that sets a target currentto be supplied to the electric motor, on the basis of the steeringtorque detected by the steering torque detector; a first motor drivecontroller including: a first proportional controller that performs aproportional action of multiplying, by a proportional gain, a valuecorresponding to a current deviation between the target current set bythe target current setting unit and the actual current detected by thecurrent detector, and also that multiplies the multiplication result bya correction factor thereby to enhance an effect of the proportionalaction; a first integral controller that performs an integral action ofmultiplying, by an integral gain, an integral value obtained byintegrating the value corresponding to the current deviation; and afirst multiplication unit that multiplies, by a factor depending on thecorrection factor, the actual current detected by the current detector,the first motor drive controller adding together an output value fromthe first proportional controller, an output value from the firstintegral controller, and an output value from the first multiplicationunit, and outputting a command value to the electric motor; a secondmotor drive controller including: a second proportional controller thatperforms a proportional action of multiplying, by the proportional gain,the value corresponding to the current deviation; a second integralcontroller that performs an integral action of multiplying, by theintegral gain, the integral value obtained by integrating the valuecorresponding to the current deviation, and also that multiplies themultiplication result by a correction factor thereby to enhance aneffect of the integral action; and a second multiplication unit thatmultiplies, by a factor depending on the correction factor, an integralvalue obtained by integrating a value of the actual current detected bythe current detector, the second motor drive controller adding togetheran output value from the second proportional controller, an output valuefrom the second integral controller, and an output value from the secondmultiplication unit, and outputting a command value to the electricmotor; and a switching unit that performs switching between the firstmotor drive controller and the second motor drive controller to outputthe command value to the electric motor.

Here, it is preferable that the switching unit perform the switching inaccordance with at least any one of a vehicle speed of a vehiclemounting the electric power steering apparatus and the steering torquedetected by the steering torque detector.

In order to attain the object, in the present invention, there isprovided an electric power steering apparatus including: a first rotaryshaft connected to a steering wheel; a rack shaft that effects turningof a wheel to be turned, by a rectilinear motion; a second rotary shaftthat effects the rectilinear motion of the rack shaft; a torsion barthat provides a connection between the first rotary shaft and the secondrotary shaft and is twisted by operation of the steering wheel; anelectric motor that applies assist force for the operation of thesteering wheel; a steering torque detector that detects steering torqueof the steering wheel; and a target current setting unit that sets atarget current to be supplied to the electric motor, on the basis of thesteering torque detected by the steering torque detector. The targetcurrent setting unit includes a resonance compensator that is providedon an output side of the steering torque detector and that suppresses aresonance frequency component of a control system including the torsionbar as a spring element, and the electric motor, the second rotary shaftand the rack shaft as inertial elements, and the target current settingunit sets the target current in accordance with the steering torquesubjected to the suppression of the resonance frequency component by theresonance compensator.

Here, it is preferable that the resonance compensator have a filteringfunction and a low-pass filtering function, the filtering functionhaving an antiresonant element of the control system.

In addition, it is preferable that a numerator of a transfer function ofthe resonance compensator have the same element as that of a denominatorof a transfer function of the control system.

Moreover, it is preferable that a denominator of the transfer functionof the resonance compensator have a degree not less than a degree of thenumerator.

From furthermore standpoint, in the present invention, there is provideda control method of an electric power steering apparatus including afirst rotary shaft connected to a steering wheel; a rack shaft thateffects turning of a wheel to be turned, by a rectilinear motion; asecond rotary shaft that effects the rectilinear motion of the rackshaft; a torsion bar that provides a connection between the first rotaryshaft and the second rotary shaft and is twisted by operation of thesteering wheel; an electric motor that applies assist force for theoperation of the steering wheel, the control method thereof including:detecting steering torque of the steering wheel; and suppressing aresonance frequency component of a control system including the torsionbar as a spring element, and the electric motor, the second rotary shaftand the rack shaft as inertial elements; and setting a target current tobe supplied to the electric motor in accordance with the steering torquesubjected to the suppression of the resonance frequency component.

Here, it is preferable that a filtering function and a low-passfiltering function be used at the suppression of the resonance frequencycomponent of the control system, the filtering function having anantiresonant element of the control system.

According to the present invention, it is capable of achieving theimprovement in the responsiveness, while suppressing an adverseinfluence on the stationary characteristics of the electric motor,without having to redesign the existing feedback control.

According to the present invention, it is capable of achieving theimprovement in the responsiveness to the actual current withoutaffecting the stability of the system.

According to the present invention, it is capable of achieving highlyaccurate suppression of occurrence of oscillation, because ofsuppressing the resonance frequency component in the control systemincluding the torsion bar as the spring element, and the electric motor,the second rotary shaft and the rack shaft as the inertial elements.Thereby, an improvement in the stability of the control system isachievable.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiment(s) of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a diagram showing an outline configuration of an electricpower steering apparatus according to the first exemplary embodiment;

FIG. 2 is a schematic configuration diagram of the control device of theelectric power steering apparatus;

FIG. 3 is a schematic configuration diagram of the target currentcalculator;

FIG. 4 is a schematic configuration diagram of the controller;

FIG. 5 is a block diagram of the feedback controller and the feedforwardcontroller;

FIG. 6 is a graph showing characteristics of the band-pass filter asemployed in the first exemplary embodiment;

FIG. 7 is a graph showing the relationship between the weighting factorα₀ and the vehicle speed;

FIG. 8 is a graph showing the relationship between the weighting factorα₀ and the amount of change in the torque of the steering wheel;

FIG. 9 is a graph showing a comparison of step response;

FIG. 10 is a diagram showing an outline configuration of an electricpower steering apparatus according to the second exemplary embodiment;

FIG. 11 is a schematic configuration diagram of the control device ofthe electric power steering apparatus;

FIG. 12 is a schematic configuration diagram of the target currentcalculator;

FIG. 13 is a schematic configuration diagram of the controller;

FIG. 14 is a graph showing the relationship between the correctionfactor α and the vehicle speed;

FIG. 15 is a simple block diagram of the controller;

FIG. 16 is a simple block diagram of a system compared to the systemaccording to the second exemplary embodiment;

FIG. 17 is a graph showing the relationship between the correctionfactor α and the amount of torque change of the steering wheel;

FIG. 18 is a schematic configuration diagram of a controller accordingto the third exemplary embodiment;

FIG. 19 is a simple block diagram of the controller according to thethird exemplary embodiment;

FIG. 20 is a schematic configuration diagram of a controller accordingto the fourth exemplary embodiment;

FIG. 21 is a schematic configuration diagram of a controller accordingto the fifth exemplary embodiment;

FIG. 22 is a simple block diagram of the controller according to thefifth exemplary embodiment;

FIG. 23 is a diagram showing an outline configuration of an electricpower steering apparatus according to the sixth exemplary embodiment;

FIG. 24 is a schematic configuration diagram of the control device ofthe electric power steering apparatus;

FIG. 25 is a schematic configuration diagram of the target currentcalculator;

FIG. 26 is a schematic configuration diagram of the controller;

FIGS. 27A and 27B are bode diagrams showing a comparison of frequencycharacteristics of the control system between the presence of theresonance compensator and the absence of the resonance compensator;

FIG. 28 is a diagram showing an outline configuration of an electricpower steering apparatus according to the seventh exemplary embodiment;

FIG. 29 is a schematic configuration diagram of the controller accordingto the seventh exemplary embodiment;

FIG. 30 is a diagram showing an outline configuration of an electricpower steering apparatus according to an eighth exemplary embodiment;

FIG. 31 is a schematic configuration diagram of the controller accordingto the eighth exemplary embodiment;

FIG. 32 is a graph showing the relationship between the correctionfactor α₁ and the amount of change in the weight of the front wheelsrelative to its reference value;

FIG. 33 is a graph showing the relationship between the correctionfactor α₁ and the vehicle speed;

FIG. 34 is a graph showing the relationship between the correctionfactor α₁ and the absolute value of the steering angle;

FIG. 35 is a graph showing the relationship between the correctionfactor β₁ and the amount of change in the weight of the front wheelsrelative to its reference value;

FIG. 36 is a graph showing the relationship between the correctionfactor β₁ and the vehicle speed; and

FIG. 37 is a graph showing the relationship between the correctionfactor β₁ and the absolute value of the steering angle.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described indetail below with reference to the accompanying drawings.

First Exemplary Embodiment

FIG. 1 is a diagram showing an outline configuration of an electricpower steering apparatus 100 according to the first exemplaryembodiment.

The electric power steering apparatus 100 (hereinafter sometimes calledmerely the “steering apparatus 100”) acts as the steering apparatus forchanging the direction of travel of a vehicle into an any direction,and, in the first exemplary embodiment, exemplifies a configuration asapplied to an automobile.

The steering apparatus 100 includes a steering wheel 101 in the form ofwheel which a driver operates, and a steering shaft 102 providedintegrally with the steering wheel 101. The steering shaft 102 and anupper connecting shaft 103 are connected together via a universalcoupling 103 a, and the upper connecting shaft 103 and a lowerconnecting shaft 108 are connected together via a universal coupling 103b.

Also, the steering apparatus 100 includes tie rods 104 connectedrespectively to right and left front wheels 150 as rolling wheels, and arack shaft 105 connected to the tie rods 104. Also, the steeringapparatus 100 includes a pinion 106 a that forms a rack-and-pinionmechanism in conjunction with rack teeth 105 a formed in the rack shaft105. The pinion 106 a is formed at a lower end portion of a pinion shaft106.

Also, the steering apparatus 100 includes a steering gear box 107 inwhich the pinion shaft 106 is housed. In the steering gear box 107, thepinion shaft 106 is connected to the lower connecting shaft 108 via atorsion bar (not shown in the figure). In addition, provided in thesteering gear box 107 is a torque sensor 109 as an example of a steeringtorque detector that detects steering torque of the steering wheel 101on the basis of a relative angle between the lower connecting shaft 108and the pinion shaft 106.

Also, the steering apparatus 100 includes an electric motor 110supported on the steering gear box 107, and a reduction gear mechanism111 that reduces drive power of the electric motor 110 and transfers thereduced drive power to the pinion shaft 106.

Also, the steering apparatus 100 includes a motor current detector 33(see FIG. 4) as an example of a current detector that detects themagnitude and direction of an actual current actually passing throughthe electric motor 110, and a motor voltage detector 160 that detects aterminal-to-terminal voltage of the electric motor 110.

The steering apparatus 100 includes a control device 10 that controlsactuation of the electric motor 110. Inputted to the control device 10are an output value from the above-mentioned torque sensor 109, anoutput value from a vehicle speed sensor 170 that detects the vehiclespeed of the automobile, an output value from the motor current detector33, and an output value from the motor voltage detector 160.

In the electric power steering apparatus 100 configured as describedabove, the steering torque applied to the steering wheel 101 is detectedby the torque sensor 109, the electric motor 110 is driven in accordancewith the detected torque, and torque produced by the electric motor 110is transmitted to the pinion shaft 106. Thereby, the torque produced bythe electric motor 110 assists the application of driver's steeringforce to the steering wheel 101.

Next, a description will be given with regard to the control device 10.

The control device 10 is an arithmetic logic circuit formed of a CPU, aROM, a RAM, a backup RAM and the like.

FIG. 2 is a schematic configuration diagram of the control device 10 ofthe electric power steering apparatus 100.

The control device 10 receives a torque signal Td obtained through theconversion of the steering torque detected by the above-mentioned torquesensor 109 into an output signal, and a vehicle speed signal v obtainedthrough the conversion of the vehicle speed detected by the vehiclespeed sensor 170 into an output signal.

Also, the control device 10 receives a motor current signal Im obtainedthrough the conversion of the actual current detected by the motorcurrent detector 33 into an output signal, and a terminal-to-terminalvoltage signal Vm of the motor, obtained through the conversion of thevoltage detected by the motor voltage detector 160 into an outputsignal.

Incidentally, since the detected signals in analog form are receivedfrom the torque sensor 109 and the like, the control device 10 uses anA/D (analog-to-digital) converter (not shown in the figure) to convertthe analog signals into digital signals and captures the digital signalsin the CPU.

The control device 10 includes a target current calculator 20 thatcalculates target assist torque on the basis of the torque signal Td andcalculates a target current required for the electric motor 110 tosupply the target assist torque, and a controller 30 that performsfeedback control or the like on the basis of the target currentcalculated by the target current calculator 20.

Next, a detailed description will be given with regard to the targetcurrent calculator 20. FIG. 3 is a schematic configuration diagram ofthe target current calculator 20.

The target current calculator 20 includes a base current calculator 21that calculates a base current for use as a reference for setting thetarget current, and an inertia compensation current calculator 22 thatcalculates a current to cancel out the moment of inertia of the electricmotor 110. Also, the target current calculator 20 includes a dampercompensation current calculator 23 that calculates a current to limitmotor revolutions, and a motor revolution speed estimation unit 24 thatestimates the revolution speed of the electric motor 110 on the basis ofthe motor current signal Im and the terminal-to-terminal voltage signalVm of the motor. Also, the target current calculator 20 includes a finaltarget current determination unit 25 that determines a final targetcurrent on the basis of outputs from the base current calculator 21, theinertia compensation current calculator 22, the damper compensationcurrent calculator 23, and so on.

The base current calculator 21 calculates the base current on the basisof a torque signal Ts obtained by a phase compensator 26 providing aphase compensation of the torque signal Td, and the vehicle speed signalv from the vehicle speed sensor 170, and outputs a base current signalIms containing information on the base current. Incidentally, thecalculation of the base current by the base current calculator 21 isaccomplished by, for example, substituting the torque signal Ts and thevehicle speed signal v into a map showing the correspondence between acombination of the torque signal Ts and the vehicle speed signal v andthe base current, which has previously been created on the basis of anempirical rule and been stored in the ROM.

The inertia compensation current calculator 22 calculates an inertiacompensation current to cancel out the moment of inertia of the electricmotor 110 and a system, on the basis of the torque signal Td and thevehicle speed signal v, and outputs an inertia compensation currentsignal Is containing information on the inertia compensation current.Incidentally, the calculation of the inertia compensation current by theinertia compensation current calculator 22 is accomplished by, forexample, substituting the torque signal Td and the vehicle speed signalv into a map showing the correspondence between a combination of thetorque signal Td and the vehicle speed signal v and the inertiacompensation current, which has previously been created on the basis ofan empirical rule and been stored in the ROM.

The damper compensation current calculator 23 calculates a dampercompensation current to limit the revolutions of the electric motor 110,on the basis of the torque signal Td, the vehicle speed signal v, and arevolution speed signal Nm of the electric motor 110, and outputs adamper compensation current signal Id containing information on thedamper compensation current. Incidentally, the calculation of the dampercompensation current by the damper compensation current calculator 23 isaccomplished by, for example, substituting the torque signal Td, thevehicle speed signal v and the revolution speed signal Nm into a mapshowing the correspondence between a combination of the torque signalTd, the vehicle speed signal v and the revolution speed signal Nm andthe damper compensation current, which has previously been created onthe basis of an empirical rule and been stored in the ROM.

The final target current determination unit 25 determines the finaltarget current on the basis of the base current signal Ims outputted bythe base current calculator 21, the inertia compensation current signalIs outputted by the inertia compensation current calculator 22, and thedamper compensation current signal Id outputted by the dampercompensation current calculator 23, and outputs a target current signalIT containing information on the final target current. The calculationof the final target current by the final target current determinationunit 25 is accomplished by, for example, substituting a compensationcurrent obtained by adding the inertia compensation current to the basecurrent and also subtracting the damper compensation current from theadded result, into a map showing the correspondence between thecompensation current and the final target current, which has previouslybeen created on the basis of an empirical rule and been stored in theROM.

As described above, the target current calculator 20 functions as anexample of a target current setting unit that sets the target current tobe supplied to the electric motor 110, on the basis of the steeringtorque detected by the torque sensor 109.

Next, a detailed description will be given with regard to the controller30. FIG. 4 is a schematic configuration diagram of the controller 30.

The controller 30 includes a motor drive controller 31 that controls theactuation of the electric motor 110, a motor drive unit 32 that drivesthe electric motor 110, and the motor current detector 33 that detectsthe actual current actually passing through the electric motor 110.

The motor drive controller 31 includes a feedback (F/B) controller 40 asan example of a feedback controller that performs feedback control onthe basis of a deviation between the target current calculated by thetarget current calculator 20 and the actual current detected by themotor current detector 33 and supplied to the electric motor 110. Also,the motor drive controller 31 includes a feedforward (F/F) controller 50as an example of a feedforward controller that performs feedforwardcontrol on the basis of the target current calculated by the targetcurrent calculator 20. The detailed description will be given later withregard to the feedback (F/B) controller 40 and the feedforward (F/F)controller 50.

Further, the motor drive controller 31 includes a pulse width modulation(PWM) signal generator 60 that generates a PWM signal to provide PWMdrive to the electric motor 110. The PWM signal generator 60 generates aPWM signal 60 a on the basis of output values from the feedforward (F/F)controller 50 and the feedback (F/B) controller 40, and outputs thegenerated PWM signal 60 a to the motor drive unit 32.

The motor drive unit 32 includes a motor drive circuit 70 formed of fourfield-effect transistors for electric power connected in theconfiguration of an H type bridge circuit, and a gate drive circuit unit80 that drives gates of two field-effect transistors selected from amongthe four field effect transistors thereby to bring the two selectedfield-effect transistors into switching operation. The gate drivecircuit unit 80 selects two field-effect transistors in accordance withthe steering direction of the steering wheel 101, on the basis of adrive control signal (the PWM signal) 60 a outputted by the PWM signalgenerator 60, and brings the two selected field-effect transistors intoswitching operation.

The motor current detector 33 detects the value of a motor current (oran armature current) passing through the electric motor 110, from avoltage between both ends of a shunt resistor 71 connected in serieswith the motor drive circuit 70, and outputs the motor current signalIm.

A description will be given with regard to the feedback controller 40and the feedforward controller 50.

FIG. 5 is a block diagram of the feedback (F/B) controller 40 and thefeedforward (F/F) controller 50.

The feedback controller 40 includes a deviation calculator 41 thatdetermines the deviation between the target current calculated by thetarget current calculator 20 and the actual current detected by themotor current detector 33, and a feedback (F/B) processor 45 thatperforms feedback processing so that the deviation would become zero.

The deviation calculator 41 outputs, as a deviation signal 41 a, thevalue of the deviation between the output value IT from the targetcurrent calculator 20 and the output value Im from the motor currentdetector 33.

The feedback (F/B) processor 45 serves to perform feedback control sothat the actual current would coincide with the target current, andgenerates and outputs a feedback processing signal 42 a by, for example,using a proportional element to perform proportional processing on theinputted deviation signal 41 a and output the proportional-processedsignal; using an integral element to perform integral processing on theinputted deviation signal 41 a and output the integral-processed signal;and using an add operation unit to add these processed signals together.

The feedforward controller 50 is provided in order to improve follow-upcharacteristics for the target current calculated by the target currentcalculator 20, and basically provides the output value in accordancewith a variation in the target current.

The feedforward controller 50 includes a feedforward (F/F) processor 51that performs feedforward processing on the target current calculated bythe target current calculator 20, and a band-pass filter (BPF) 52 as anexample of a frequency compensator that effects a change in afeedforward effect in accordance with the frequency of a signaloutputted by the feedforward (F/F) processor 51.

Further, the feedforward controller 50 includes a weighting processor 53as an example of a weighting processor that performs weighting on anoutput value from the band-pass filter 52, and a weighting factorsetting unit 54 as an example of a weighting factor setting unit thatsets a weighting factor α₀ used for the weighting processor 53 toperform processing.

Further, the feedforward controller 50 includes an adder 55 that addstogether an output value 53 a from the weighting processor 53 and theoutput value 42 a from the feedback controller 40 and outputs aresultant value 50 a to the PWM signal generator 60.

The feedforward (F/F) processor 51 performs processing such that itstransfer function is given as an inverse function of a transfer functionof the electric motor 110. In short, the transfer function H(s) of thefeedforward processor 51 is given as H(s)=1/G(s), where G(s) representsthe transfer function of the electric motor 110. For instance, if thetransfer function G(s) of the electric motor 110 is expressed byEquation (1) below, the transfer function H(s) of the feedforwardprocessor 51 is expressed by Equation (2) below:

G(s)=Jm×s/(Jm×L×s ² +Jm×R×s+Ke×Kt)  (1)

H(s)=(Jm×L×s ² +Jm×R×s+Ke×Kt)/(Jm×s)  (2)

where Jm denotes the inertia of a motor shaft of the electric motor 110;Kt, a motor torque constant of the electric motor 110; and Ke, aninduction voltage constant of the electric motor 110. Incidentally, sdenotes an operator for a Laplace transform.

The band-pass filter (BPF) 52 is the filter that transmits a requiredrange of frequencies alone but does not transmit (or attenuates)frequencies outside this range. FIG. 6 is a graph showingcharacteristics of the band-pass filter 52 as employed in the firstexemplary embodiment. The band-pass filter 52 according to the firstexemplary embodiment has a transmission region of frequencies between f1and f2 but does not transmit or attenuates frequencies outside thisregion, as shown in FIG. 6. In a region of lower frequencies than f1,therefore, the output value from the feedforward processor 51 attenuatesmore as the frequency gets lower. In a region of higher frequencies thanf2, also, the output value from the feedforward processor 51 attenuatesmore as the frequency gets higher.

Incidentally, f1 and f2 may be set at 5 Hz and 100 Hz, respectively, byway of example. Setting f2 at 100 Hz permits the suppression of theadverse influence of noise components upon steer feeling of the steeringwheel 101, even if noise is contained in the torque signal Tdorthe like.Incidentally, at least any one of a low-pass filter (LPF) and ahigh-pass filter (HPF) may be provided as appropriate in place of theband-pass filter (BPF) 52.

The weighting processor 53 performs the processing of multiplying theoutput value from the band-pass filter 52 by the weighting factor α₀ setby the weighting factor setting unit 54.

The weighting factor setting unit 54 calculates the weighting factor α₀on the basis of the vehicle speed signal v. FIG. 7 is a graph showingthe relationship between the weighting factor α₀ and the vehicle speed.The optimum weighting factor α₀ for the vehicle speed is derived inadvance on the basis of an empirical rule, as shown in FIG. 7. Then, theweighting factor setting unit 54 calculates and sets the weightingfactor α₀ by substituting the vehicle speed signal v into a map showingthe correspondence between the vehicle speed signal v and the weightingfactor α₀, or a relational expression of the vehicle speed signal v andthe weighting factor α₀, which has previously been created and stored inthe ROM.

Also, the weighting factor setting unit 54 may calculate the weightingfactor α₀ on the basis of the torque signal Td. FIG. 8 is a graphshowing the relationship between the weighting factor α₀ and the amountof change in the torque of the steering wheel 101. The optimum weightingfactor α₀ for the amount of change in the torque of the steering wheel101 is derived in advance on the basis of an empirical rule, as shown inFIG. 8. Then, the weighting factor setting unit 54 calculates theweighting factor α₀ by substituting the amount of torque change derivedfrom the torque signal Td into a map showing the correspondence betweenthe amount of change in the torque of the steering wheel 101 and theweighting factor α₀, which has previously been created and stored in theROM. Alternatively, the calculation of the weighting factor α₀ may beaccomplished by substituting the amount of torque change into arelational expression of the amount of torque change and the weightingfactor α₀, which has previously been created. Incidentally, adifferential value of the torque signal Td may be calculated as theamount of torque change.

Also, it is preferable that the weighting factor setting unit 54calculate the weighting factor α₀ on the basis of the vehicle speedsignal v and the torque signal Td. Also in such an instance, therelationship between a combination of the vehicle speed signal v and theamount of change in the torque of the steering wheel 101 and the optimumweighting factor α₀ is derived in advance on the basis of an empiricalrule. Then, a map showing the correspondence therebetween is created andstored in the ROM in advance, and the weighting factor setting unit 54calculates the weighting factor α₀ by substituting the vehicle speedsignal v and the amount of torque change into the map. Alternatively,the calculation of the weighting factor α₀ may be accomplished bysubstituting the vehicle speed signal v and torque variation into arelational expression of a combination of the vehicle speed signal v andthe amount of torque change and the weighting factor α₀, which haspreviously been created.

A description will be given below with regard to function of theelectric power steering apparatus 100 configured as described above.

FIG. 9 is a graph showing a comparison of step response.

In FIG. 9, the step response to the output value 50 a from thefeedforward controller 50 and the feedback (F/B) controller 40 of thecontroller 30 according to the first exemplary embodiment is shown bythe solid line, provided that the target current calculated by thetarget current calculator 20 is taken as a step function. Also, the stepresponse to the output value 50 a in a system (a first comparativesystem) in which the band-pass filter 52, the weighting processor 53 andthe weighting factor setting unit 54 are excluded from the feedforwardcontroller 50 according to the first exemplary embodiment is shown bythe dashed dotted line. Also, the step response to the output value fromthe feedback controller 40 in a system (a second comparative system) inwhich the feedforward controller 50 is excluded from the controller 30according to the first exemplary embodiment, that is, the feedbackcontroller 40 alone is provided, is shown by the dash line. These stepresponses are compared to the step response to the target current shownby the solid thick line (the thick line).

As shown in FIG. 9, the controller 30 according to the first exemplaryembodiment, because of being provided with the feedforward controller50, has better responsiveness than the second comparative systemprovided with the feedback controller 40 alone. Also, in a steady state,the feedforward controller 50 of the controller 30 according to thefirst exemplary embodiment is closer to the target current than thefirst comparative system. This is due to the fact that because theband-pass filter 52 cancels or attenuates the output value in a lowerfrequency range, the feedforward effect is also canceled or attenuated.In other words, by the band-pass filter 52, the amount of increase inthe actual current by the feedforward controller 50 becomes smaller, asthe frequency of variation in the target current is lower.

Therefore, for example, when the steering wheel 101 makes a transitionfrom a state in which it is held in a given position to a rotatingstate, the output value from the feedforward controller 50 varies,following a variation in the output value IT from the target currentcalculator 20. Hence, an input value to the PWM signal generator 60 islarger than that of the system provided with the feedback controller 40alone without the provision of the feedforward controller 50, which inturn achieves an improvement in responsiveness to operation of thesteering wheel 101.

Also, the weighting factor α₀ is determined in accordance with thevehicle speed as shown in FIG. 7, thereby to achieve an improvement inthe responsiveness to the operation of the steering wheel 101 underlow-speed driving such as a situation where the automobile is put in agarage. On the other hand, under high-speed driving, the responsivenessis reduced, and thus, the operation of the steering wheel 101 becomesstabilized.

Also, the weighting factor α₀ is determined in accordance with thetorque variation as shown in FIG. 8, thereby to achieve an improvementin the responsiveness to the operation of the steering wheel 101 whenthe operation of the steering wheel 101 is relatively slowly performed,such as when the automobile is put in the garage. On the other hand,under abrupt steering, the responsiveness is reduced, and thus, safetyis provided.

On the other hand, for example, when the steering wheel 101 is held in agiven position or is slowly operated, the band-pass filter 52 cancels orattenuates the feedforward effect. In other words, by the band-passfilter 52, the amount of increase in the actual current by thefeedforward controller 50 becomes smaller, as the frequency of variationin the target current is lower. Hence, the input value to the PWM signalgenerator 60 is the same as that of the system provided with thefeedback controller 40 alone without the provision of the feedforwardcontroller 50, or does not increase greatly even if increased, and thusis stable.

As described above, the electric power steering apparatus 100 accordingto the first exemplary embodiment is capable of achieving theimprovement in the responsiveness, while suppressing an adverseinfluence on stationary characteristics of the electric motor 110. Also,the addition of the feedforward controller 50 according to the firstexemplary embodiment to the controller provided with the feedbackcontroller 40 alone permits achieving the above-mentioned effect withouthaving to redesign the feedback controller 40.

Second Exemplary Embodiment

FIG. 10 is a diagram showing an outline configuration of an electricpower steering apparatus 100 according to the second exemplaryembodiment.

The electric power steering apparatus 100 (hereinafter sometimes calledmerely the “steering apparatus 100”) acts as the steering apparatus forchanging the direction of travel of a vehicle into an any direction,and, in the second exemplary embodiment, exemplifies a configuration asapplied to an automobile.

The steering apparatus 100 includes a steering wheel 101 in the form ofwheel which a driver operates, and a steering shaft 102 providedintegrally with the steering wheel 101. The steering shaft 102 and anupper connecting shaft 103 are connected together via a universalcoupling 103 a, and the upper connecting shaft 103 and a lowerconnecting shaft 108 are connected together via a universal coupling 103b.

Also, the steering apparatus 100 includes tie rods 104 connectedrespectively to right and left front wheels 150 as rolling wheels, and arack shaft 105 connected to the tie rods 104. Also, the steeringapparatus 100 includes a pinion 106 a that forms a rack-and-pinionmechanism in conjunction with rack teeth 105 a formed in the rack shaft105. The pinion 106 a is formed at a lower end portion of a pinion shaft106.

Also, the steering apparatus 100 includes a steering gear box 107 inwhich the pinion shaft 106 is housed. In the steering gear box 107, thepinion shaft 106 is connected to the lower connecting shaft 108 via atorsion bar (not shown in the figure). In addition, provided in thesteering gear box 107 is a torque sensor 109 as an example of a steeringtorque detector that detects steering torque of the steering wheel 101on the basis of a relative angle between the lower connecting shaft 108and the pinion shaft 106.

Also, the steering apparatus 100 includes an electric motor 110supported on the steering gear box 107, and a reduction gear mechanism111 that reduces drive power of the electric motor 110 and transfers thereduced drive power to the pinion shaft 106.

Also, the steering apparatus 100 includes a motor current detector 33(see FIG. 13) as an example of a current detector that detects themagnitude and direction of an actual current actually passing throughthe electric motor 110, and a motor voltage detector 160 that detects aterminal-to-terminal voltage of the electric motor 110.

The steering apparatus 100 includes a control device 10 that controlsactuation of the electric motor 110. Inputted to the control device 10are an output value from the above-mentioned torque sensor 109, anoutput value from a vehicle speed sensor 170 that detects the vehiclespeed of the automobile, an output value from the motor current detector33, and an output value from the motor voltage detector 160.

In the electric power steering apparatus 100 configured as describedabove, the steering torque applied to the steering wheel 101 is detectedby the torque sensor 109, the electric motor 110 is driven in accordancewith the detected torque, and torque produced by the electric motor 110is transmitted to the pinion shaft 106. Thereby, the torque produced bythe electric motor 110 assists the application of driver's steeringforce to the steering wheel 101.

Next, a description will be given with regard to the control device 10.

The control device 10 is an arithmetic logic circuit formed of a CPU, aROM, a RAM, a backup RAM and the like.

FIG. 11 is a schematic configuration diagram of the control device 10 ofthe electric power steering apparatus 100.

The control device 10 receives a torque signal Td obtained through theconversion of the steering torque detected by the above-mentioned torquesensor 109 into an output signal, and a vehicle speed signal v obtainedthrough the conversion of the vehicle speed detected by the vehiclespeed sensor 170 into an output signal.

Also, the control device 10 receives a motor current signal Im obtainedthrough the conversion of the actual current detected by the motorcurrent detector 33 into an output signal, and a terminal-to-terminalvoltage signal Vm of the motor, obtained through the conversion of thevoltage detected by the motor voltage detector 160 into an outputsignal.

Incidentally, since the detected signals in analog form are receivedfrom the torque sensor 109 and the like, the control device 10 uses anA/D (analog-to-digital) converter (not shown in the figure) to convertthe analog signals into digital signals and captures the digital signalsin the CPU.

The control device 10 includes a target current calculator 20 thatcalculates target assist torque on the basis of the torque signal Td andcalculates a target current required for the electric motor 110 tosupply the target assist torque, and a controller 30 that performsfeedback control or the like on the basis of the target currentcalculated by the target current calculator 20.

Next, a detailed description will be given with regard to the targetcurrent calculator 20. FIG. 12 is a schematic configuration diagram ofthe target current calculator 20.

The target current calculator 20 includes a base current calculator 21that calculates a base current for use as a reference for setting thetarget current, and an inertia compensation current calculator 22 thatcalculates a current to cancel out the moment of inertia of the electricmotor 110. Also, the target current calculator 20 includes a dampercompensation current calculator 23 that calculates a current to limitmotor revolutions, and a motor revolution speed estimation unit 24 thatestimates the revolution speed of the electric motor 110 on the basis ofthe motor current signal Im and the terminal-to-terminal voltage signalVm of the motor. Also, the target current calculator 20 includes a finaltarget current determination unit 25 that determines a final targetcurrent on the basis of outputs from the base current calculator 21, theinertia compensation current calculator 22, the damper compensationcurrent calculator 23, and so on.

The base current calculator 21 calculates the base current on the basisof a torque signal Ts obtained by a phase compensator 26 providing aphase compensation of the torque signal Td, and the vehicle speed signalv from the vehicle speed sensor 170, and outputs a base current signalIms containing information on the base current. Incidentally, thecalculation of the base current by the base current calculator 21 isaccomplished by, for example, substituting the torque signal Ts and thevehicle speed signal v into a map showing the correspondence between acombination of the torque signal Ts and the vehicle speed signal v andthe base current, which has previously been created on the basis of anempirical rule and been stored in the ROM.

The inertia compensation current calculator 22 calculates an inertiacompensation current to cancel out the moment of inertia of the electricmotor 110 and a system, on the basis of the torque signal Td and thevehicle speed signal v, and outputs an inertia compensation currentsignal Is containing information on the inertia compensation current.Incidentally, the calculation of the inertia compensation current by theinertia compensation current calculator 22 is accomplished by, forexample, substituting the torque signal Td and the vehicle speed signalv into a map showing the correspondence between a combination of thetorque signal Td and the vehicle speed signal v and the inertiacompensation current, which has previously been created on the basis ofan empirical rule and been stored in the ROM.

The damper compensation current calculator 23 calculates a dampercompensation current to limit the revolutions of the electric motor 110,on the basis of the torque signal Td, the vehicle speed signal v, and arevolution speed signal Nm of the electric motor 110, and outputs adamper compensation current signal Id containing information on thedamper compensation current. Incidentally, the calculation of the dampercompensation current by the damper compensation current calculator 23 isaccomplished by, for example, substituting the torque signal Td, thevehicle speed signal v and the revolution speed signal Nm into a mapshowing the correspondence between a combination of the torque signalTd, the vehicle speed signal v and the revolution speed signal Nm andthe damper compensation current, which has previously been created onthe basis of an empirical rule and been stored in the ROM.

The final target current determination unit 25 determines the finaltarget current on the basis of the base current signal Ims outputted bythe base current calculator 21, the inertia compensation current signalIs outputted by the inertia compensation current calculator 22, and thedamper compensation current signal Id outputted by the dampercompensation current calculator 23, and outputs a target current signalIT containing information on the final target current. The calculationof the final target current by the final target current determinationunit 25 is accomplished by, for example, substituting a compensationcurrent obtained by adding the inertia compensation current to the basecurrent and also subtracting the damper compensation current from theadded result, into a map showing the correspondence between thecompensation current and the final target current, which has previouslybeen created on the basis of an empirical rule and been stored in theROM.

As described above, the target current calculator 20 functions as anexample of a target current setting unit that sets the target current tobe supplied to the electric motor 110, on the basis of the steeringtorque detected by the torque sensor 109.

Next, a detailed description will be given with regard to the controller30. FIG. 13 is a schematic configuration diagram of the controller 30.

The controller 30 includes a motor drive controller 31 that controls theactuation of the electric motor 110, a motor drive unit 32 that drivesthe electric motor 110, and the motor current detector 33 that detectsthe actual current actually passing through the electric motor 110.

The motor drive controller 31 includes a feedback (F/B) controller 40that performs the feedback control on the basis of the deviation betweenthe target current calculated by the target current calculator 20 andthe actual current detected by the motor current detector 33 andsupplied to the electric motor 110. Also, the motor drive controller 31includes a multiplier 56 that multiplies, by a factor, the actualcurrent detected by the motor current detector 33. A detaileddescription will be given later with regard to the feedback (F/B)controller 40 and the multiplier 56.

Further, the motor drive controller 31 includes a pulse width modulation(PWM) signal generator 60 that generates a PWM signal to provide PWMdrive to the electric motor 110. The PWM signal generator 60 generates aPWM signal 60 a on the basis of an output value from the feedbackcontroller 40, and outputs the generated PWM signal 60 a to the motordrive unit 32.

The motor drive unit 32 includes a motor drive circuit 70 formed of fourfield-effect transistors for electric power connected in theconfiguration of an H type bridge circuit, and a gate drive circuit unit80 that drives gates of two field-effect transistors selected from amongthe four field effect transistors thereby to bring the two selectedfield-effect transistors into switching operation. The gate drivecircuit unit 80 selects two field-effect transistors in accordance withthe steering direction of the steering wheel 101, on the basis of adrive control signal (the PWM signal 60 a) outputted by the PWM signalgenerator 60, and brings the two selected field-effect transistors intoswitching operation.

The motor current detector 33 detects the value of a motor current (oran armature current) passing through the electric motor 110, from avoltage between both ends of a shunt resistor 71 connected in serieswith the motor drive circuit 70, and outputs the motor current signalIm.

Next, a description will be given with regard to the feedback controller40 and the multiplier 56.

The feedback controller 40 includes a deviation calculator 41 thatdetermines the deviation between the target current calculated by thetarget current calculator 20 and the actual current detected by themotor current detector 33. Also, the feedback controller 40 includes aproportional controller 42 that performs a proportional action on thecurrent deviation calculated by the deviation calculator 41, an integralcontroller 43 that performs an integral action on the current deviationcalculated by the deviation calculator 41, and an adder 44 that addstogether an output value from the proportional controller 42 and anoutput value from the integral controller 43.

The deviation calculator 41 outputs the value of the deviation betweenthe output value IT from the target current calculator 20 and the outputvalue Im from the motor current detector 33.

The proportional controller 42 functions as an example of a proportionalcontroller, and includes a proportional action element 421 thatmultiplies, by proportional gain Kp, the deviation between the outputvalue IT from the target current calculator 20 and the output value Imfrom the motor current detector 33. Also, the proportional controller 42includes a correction unit 422 as an example of a correction unit thatmultiplies an output value from the proportional action element 421 by acorrection factor α, and a correction factor setting unit 423 that setsthe correction factor α used for the correction unit 422 to performprocessing.

The correction factor setting unit 423 calculates the correction factorα, for example, on the basis of the vehicle speed signal v. FIG. 14 is agraph showing the relationship between the correction factor α and thevehicle speed. The optimum correction factor α in accordance with thevehicle speed is derived in advance on the basis of an empirical rule,as shown in FIG. 14. Then, the correction factor setting unit 423calculates and sets the correction factor α by substituting the vehiclespeed signal v into a map showing the correspondence between the vehiclespeed signal v and the correction factor α, or the relational expressionof the vehicle speed signal v and the correction factor α, which haspreviously been created and been stored in the ROM. Incidentally, asshown in FIG. 14, the correction factor α is 1.2 when the vehicle speedis zero, and preferably, the correction factor α decreases to 1 as thevehicle speed becomes higher, and the correction factor α is 1 when thevehicle speed is equal to or more than a given speed.

The correction factor setting unit 423 sets the correction factor α at avalue equal to or more than 1, and thereby, the correction unit 422multiplies the output value from the proportional action element 421 bythe correction factor α, thereby to enhance the effect of theproportional action performed by the proportional action element 421.

The integral controller 43 functions as an example of an integralcontroller, and is formed of an integral action element 431 thatperforms the integral action of multiplying, by integral gain Ki, theintegral value obtained by integrating the deviation between the outputvalue IT from the target current calculator 20 and the output value Imfrom the motor current detector 33.

The adder 44 functions as an example of an addition unit that addstogether the output value from the proportional controller 42, theoutput value from the integral controller 43 and the output value fromthe multiplier 56, and outputs the addition result. The output valuefrom the adder 44 is a base of a command value to the electric motor110, and the PWM signal generator 60 generates the PWM signal 60 a onthe basis of the output value from the adder 44 and outputs thegenerated PWM signal 60 a to the motor drive unit 32.

The multiplier 56 calculates a proportional factor α on the basis of thecorrection factor α set by the correction factor setting unit 423,multiplies, by the proportional gain Kp and the proportional factor α,the actual current detected by the motor current detector 33, andoutputs the multiplication result. The proportional factor α is thefactor depending on the correction factor α, and is a value obtained bysubtracting 1 from the correction factor α, that is, α−1. Hence, themultiplier 56 multiplies, by “Kp×(α−1),” the actual current detected bythe motor current detector 33, and outputs the multiplication result.

The multiplier 56 functions as an adjusting unit that makes anadjustment so that a denominator of the transfer function would remainconstant regardless of the value of the correction factor α, when thetarget current set by the target current calculator 20 is taken as aninput and the actual current actually supplied to the electric motor 110is taken as an output. This is proved by Equation (101) given below.

FIG. 15 is a simple block diagram of the controller 30. As shown in FIG.15, IT(s) represents Laplace transform of the output value IT from thetarget current calculator 20 (see FIG. 13), Im(s) represents Laplacetransform of the output value Im from the motor current detector 33, andP (s) represents in simple form a transfer function of the PWM signalgenerator 60, the motor drive unit 32 and the electric motor 110.

A transfer function H(s) from IT(s) to Im(s) is expressed by Equation(101).

$\begin{matrix}{{H(s)} = \frac{{\alpha \; s} + \left( {{Ki}/{Kp}} \right)}{\left( {1 + {1/\left( {{P(s)} \times {Kp}} \right)}} \right) + s + \left( {{Ki}/{Kp}} \right)}} & (101)\end{matrix}$

As expressed by Equation (101), a denominator of the transfer functionH(s) is not affected by the correction factor α.

FIG. 16 is a simple block diagram of a system compared to the systemaccording to the second exemplary embodiment. The system in which thecorrection unit 422 and the multiplier 56 are excluded from the motordrive controller 31 according to the second exemplary embodiment isexemplified as the system compared to the system according to the secondexemplary embodiment. FIG. 16 is a block diagram of the comparativesystem. As in the case of FIG. 15, IT(s) represents the Laplacetransform of the output value IT from the target current calculator 20,Im(s) represents the Laplace transform of the output value Im from themotor current detector 33, and P(s) represents in simple form thetransfer function of the PWM signal generator 60, the motor drive unit32 and the electric motor 110.

In this instance, a transfer function G(s) from IT(s) to Im(s) isexpressed by Equation (102).

$\begin{matrix}{{G(s)} = \frac{s + \left( {{Ki}/{Kp}} \right)}{{\left( {1 + {1/\left( {{P(s)} \times {Kp}} \right)}} \right)s} + \left( {{Ki}/{Kp}} \right)}} & (102)\end{matrix}$

When an input is IT(s) and an output is Im(s), a numerator of thetransfer function indicates the responsiveness of the actual current tothe target current. Thus, as can be seen from Equations (101) and (102),the motor drive controller 31 according to the second exemplaryembodiment enhances the effect of the proportional action performed bythe proportional action element 421 in the correction unit 422 andcorrespondingly improves the responsiveness, as compared to thecomparative system.

Also, a denominator of the transfer function indicates the stability ofthe system, and as can be seen from Equations (101) and (102), thedenominator of H(s) is the same as that of G(s). Thus, the motor drivecontroller 31 according to the second exemplary embodiment ensures thesame stability as the comparative system, regardless of the value of α.

Therefore, as is the case with the motor drive controller 31 accordingto the second exemplary embodiment, the correction unit 422 thatenhances the effect of the proportional action performed by theproportional action element 421 is provided in a part of theproportional controller 42, and also, the adder 44 adds the valueobtained by multiplying Im by the proportional gain Kp and theproportional factor β (=α−1) to the output values from the proportionalcontroller 42 and the integral controller 43, and thereby, animprovement in the responsiveness is achievable without affecting thestability of the system. Thereby, an improvement in steering feel isachievable without affecting the stability of the electric powersteering apparatus 100. Also, the correction factor α may be changed,for example, in accordance with the vehicle speed thereby to performfine control.

If the existing system is the comparative system, the system may bemodified as is the case with the motor drive controller 31 according tothe second exemplary embodiment. Thereby, the same method as theexisting system is used to set the proportional gain Kp and the integralgain Ki and thereby ensure the stability of the system, and then, thecorrection factor α is adjusted to improve the responsiveness of theactual current to the target current.

Incidentally, the correction factor setting unit 423 may calculate thecorrection factor α on the basis of the steering torque. FIG. 17 is agraph showing the relationship between the correction factor α and theamount of torque change of the steering wheel 101. For example, theoptimum correction factor α in accordance with the amount of torquechange of the steering wheel 101 is derived on the basis of an empiricalrule, as shown in FIG. 17. Then, the correction factor setting unit 423calculates the correction factor α by substituting the amount of torquechange derived from the torque signal Td into a map showing thecorrespondence between the amount of torque change of the steering wheel101 and the correction factor α, which has previously been created andbeen stored in the ROM. Alternatively, the correction factor settingunit 423 may calculate the correction factor α by substituting theamount of torque change into the relational expression of the amount oftorque change and the correction factor α, which has previously beencreated.

Also, it is preferable that the correction factor setting unit 423calculate the correction factor α on the basis of the vehicle speed andthe steering torque. For example, the relationship between a combinationof the vehicle speed signal v and the amount of torque change of thesteering wheel 101 and the optimum correction factor α is derived inadvance on the basis of an empirical rule. Then, a map showing thecorrespondence therebetween is previously created and stored in the ROM,and the correction factor setting unit 423 calculates the correctionfactor α by substituting the vehicle speed signal v and the amount oftorque change into the map. Alternatively, the correction factor settingunit 423 may calculate the correction factor α by substituting thevehicle speed signal v and the amount of torque change into therelational expression of a combination of the vehicle speed signal v andthe amount of torque change and the correction factor α, which haspreviously been created.

As described above, a change in the correction factor α on the basis ofat least any one of the vehicle speed and the torque signal Td permitsfiner control.

Third Exemplary Embodiment

FIG. 18 is a schematic configuration diagram of a controller 200according to the third exemplary embodiment.

Hereinafter, description will be given with regard to the differencebetween the third exemplary embodiment and the second exemplaryembodiment. The same components are denoted by the same referencenumerals, and the detailed description thereof will be omitted.

A feedback controller 211 of a motor drive controller 210 of thecontroller 200 according to the third exemplary embodiment includes aproportional controller 220 and an integral controller 230, as is thecase with the controller according to the second exemplary embodiment.The proportional controller 220 and the integral controller 230 areconfigured as given below. Specifically, the proportional controller 220according to the third exemplary embodiment is formed of a proportionalaction element 221 that multiplies, by the proportional gain Kp, thedeviation between the output value IT from the target current calculator20 and the output value Im from the motor current detector 33. Theintegral controller 230 includes an integral action element 231 thatperforms the integral action of multiplying, by the integral gain Ki,the integral value obtained by integrating the deviation between IT andIm, a correction unit 232 as an example of a correction unit thatmultiplies an output value from the integral action element 231 by acorrection factor γ, and a correction factor setting unit 233 that setsthe correction factor γ used for the correction unit 232 to performprocessing.

Also, the motor drive controller 210 of the controller 200 according tothe third exemplary embodiment includes a multiplier 250 thatmultiplies, by a factor, the integral value obtained by integrating thevalue of the actual current detected by the motor current detector 33.

The correction factor setting unit 233 calculates the correction factorγ, for example on the basis of the vehicle speed. For example, theoptimum correction factor γ in accordance with the vehicle speed isderived in advance on the basis of an empirical rule, as shown in FIG.14. Then, the correction factor setting unit 233 calculates and sets thecorrection factor γ by substituting the vehicle speed signal v into amap showing the correspondence between the vehicle speed signal v andthe correction factor γ, or the relational expression of the vehiclespeed signal v and the correction factor γ, which has previously beencreated and been stored in the ROM. Incidentally, as shown in FIG. 14,the correction factor γ is 1.2 when the vehicle speed is zero, andpreferably, the correction factor γ decreases to 1 as the vehicle speedbecomes higher, and the correction factor γ is 1 when the vehicle speedis equal to or more than a given speed.

The correction factor setting unit 233 sets the correction factor γ at avalue equal to or more than 1, and thereby, the correction unit 232multiplies the output value from the integral action element 231 by thecorrection factor γ, thereby to enhance the effect of the integralaction performed by the integral action element 231.

The multiplier 250 calculates a proportional factor δ on the basis ofthe correction factor γ set by the correction factor setting unit 233,multiplies, by the integral gain Ki and the proportional factor δ, theintegral value obtained by integrating the value of the actual currentdetected by the motor current detector 33, and outputs themultiplication result. The proportional factor δ is the factor dependingon the correction factor γ, and is a value obtained by subtracting 1from the correction factor γ, that is, γ−1. Hence, the multiplier 250multiplies, by “Ki×(γ−1),” the integral value obtained by integratingthe value of the actual current detected by the motor current detector33, and outputs the multiplication result.

The adder 240 functions as an example of an addition unit that addstogether the output value from the proportional controller 220, theoutput value from the integral controller 230, and the output value fromthe multiplier 250, and outputs the addition result. The output valuefrom the adder 240 is a base of a command value to the electric motor110, and the PWM signal generator 60 generates the PWM signal 60 a onthe basis of the output value from the adder 240 and outputs thegenerated PWM signal 60 a.

In the motor drive controller 210 according to the third exemplaryembodiment configured as described above, the multiplier 250 functionsas an adjusting unit that makes an adjustment so that a denominator ofthe transfer function would remain constant regardless of the value ofthe correction factor γ, when the target current set by the targetcurrent calculator 20 is taken as an input and the actual currentactually supplied to the electric motor 110 is taken as an output. Thisis proved by Equation (103) given below.

FIG. 19 is a simple block diagram of the controller 200 according to thethird exemplary embodiment. In FIG. 19, as in the case of FIG. 15, IT(s)represents the Laplace transform of the output value IT from the targetcurrent calculator 20 (see FIG. 13), Im(s) represents the Laplacetransform of the output value Im from the motor current detector 33, andP(s) represents in simple form the transfer function of the PWM signalgenerator 60, the motor drive unit 32 and the electric motor 110.

In this instance, a transfer function F(s) from IT(s) to Im(s) isexpressed by Equation (103).

$\begin{matrix}{{F(s)} = \frac{s + {\gamma \left( {{Ki}/{Kp}} \right)}}{{\left( {1 + {1/\left( {{P(s)} \times {Kp}} \right)}} \right)s} + \left( {{Ki}/{Kp}} \right)}} & (103)\end{matrix}$

As expressed by Equation (103), a denominator of the transfer function F(s) is not affected by the correction factor γ.

When an input is IT(s) and an output is Im(s), a numerator of thetransfer function indicates the responsiveness of the actual current tothe target current. Thus, as can be seen from Equations (102) and (103),the motor drive controller 210 according to the third exemplaryembodiment enhances the effect of the integral action performed by theintegral action element 231 in the correction unit 232 andcorrespondingly improves the responsiveness, as compared to thecomparative system.

Also, a denominator of the transfer function indicates the stability ofthe system, and as can be seen from Equations (102) and (103), thedenominator of F(s) is the same as that of G(s). Thus, the motor drivecontroller 210 according to the third exemplary embodiment ensures thesame stability as the comparative system, regardless of the value of γ.

Therefore, as is the case with the motor drive controller 210 accordingto the third exemplary embodiment, the correction unit 232 that enhancesthe effect of the integral action performed by the integral actionelement 231 is provided in a part of the integral controller 230, andalso, the adder 240 adds the value obtained by multiplying, by theintegral gain Ki and the proportional factor δ (=γ−1), an integral valueobtained by integrating Im, to the output values from the proportionalcontroller 220 and the integral controller 230, and thereby, animprovement in the responsiveness is achievable without affecting thestability of the system. Thereby, an improvement in steering feel isachievable without affecting the stability of the electric powersteering apparatus 100. Also, the correction factor γ may be changed,for example, in accordance with the vehicle speed thereby to performfine control.

If the existing system is the comparative system, the system may bemodified as is the case with the motor drive controller 210 according tothe third exemplary embodiment. Thereby, the same method as the existingsystem is used to set the proportional gain Kp and the integral gain Kiand thereby ensure the stability of the system, and then, the correctionfactor γ is adjusted to improve the responsiveness of the actual currentto the target current.

Incidentally, the correction factor setting unit 233 may calculate thecorrection factor γ on the basis of the steering torque, as has beendescribed in the section on the second exemplary embodiment. Also, thecorrection factor setting unit 233 may calculate the correction factor γon the basis of the vehicle speed and the steering torque, as has beendescribed in the section on the second exemplary embodiment.

As described above, a change in the correction factor γ based on atleast any one of the vehicle speed and the steering torque permits finercontrol.

Fourth Exemplary Embodiment

FIG. 20 is a schematic configuration diagram of a controller 300according to the fourth exemplary embodiment.

Hereinafter, a description will be given with regard to the differencebetween the fourth exemplary embodiment and the second or thirdexemplary embodiment. The same components are denoted by the samereference numerals, and the detailed description thereof will beomitted.

The controller 300 according to the fourth exemplary embodiment includesa first motor drive controller 311, a second motor drive controller 312,and a switching unit 310 that performs switching between the first motordrive controller 311 and the second motor drive controller 312 to outputa command value to the PWM signal generator 60 for the output value ITfrom the target current calculator 20 (see FIG. 13), according to thecircumstances.

As shown in FIG. 20, the first motor drive controller 311 has the sameconfiguration and function as the motor drive controller 31 according tothe second exemplary embodiment (for instance, the first motor drivecontroller 311 has the multiplier 56 as an example of a firstmultiplication unit), and the second motor drive controller 312 has thesame configuration and function as the motor drive controller 210according to the third exemplary embodiment (for instance, the secondmotor drive controller 312 has the multiplier 250 as an example of asecond multiplication unit).

The switching unit 310 selects either the first motor drive controller311 or the second motor drive controller 312 according to thecircumstances.

Here, when the switching unit 310 selects the first motor drivecontroller 311, a transfer function from IT(s) to Im(s) is H(s)represented as Equation (101), or when the switching unit 310 selectsthe second motor drive controller 312, the transfer function from IT(s)to Im(s) is F(s) represented as Equation (103).

Because of characteristics of H(s) and F(s), the actual current reachesthe target current when the switching unit 310 selects the first motordrive controller 311 faster than when the switching unit 310 selects thesecond motor drive controller 312. On the other hand, the actual currentconverges on the target current when the switching unit 310 selects thesecond motor drive controller 312 faster than when the switching unit310 selects the first motor drive controller 311.

It is therefore preferable that the switching unit 310 perform switchingbetween the motor drive controllers in accordance with the vehiclespeed, in such a manner that the actual current reaches the targetcurrent faster, if the vehicle speed is low, or in such a manner thatthe actual current converges on the target current faster in order tosuppress vibration corresponding to the vehicle speed, if the vehiclespeed is high. Specifically, it is preferable that the switching unit310 perform switching so as to select the first motor drive controller311 if the vehicle speed detected by the vehicle speed sensor 170 isequal to or less than a threshold value, or select the second motordrive controller 312 if the vehicle speed is more than the thresholdvalue.

This permits finer control of the electric motor 110, thus achieving afurther improvement in the steering feel without affecting the stabilityof the electric power steering apparatus 100.

Also, it is preferable that the switching unit 310 perform switching,allowing for the steering torque detected by the torque sensor 109. Forexample, the relationship between a combination of the vehicle speed andthe steering torque and the optimum motor drive controller is derived inadvance on the basis of an empirical rule. Then, a map showing thecorrespondence therebetween is created and stored in advance in the ROM,and the switching unit 310 substitutes the vehicle speed signal v andthe torque signal Td into the map thereby to select either one of themotor drive controllers. This permits finer control of the electricmotor 110.

Fifth Exemplary Embodiment

FIG. 21 is a schematic configuration diagram of a controller 400according to the fifth exemplary embodiment.

Hereinafter, a description will be given with regard to the differencebetween the fifth exemplary embodiment and the second exemplaryembodiment. The same components are denoted by the same referencenumerals, and the detailed description thereof will be omitted.

A feedback controller 440 of a motor drive controller 410 of thecontroller 400 according to the fifth exemplary embodiment includes theproportional controller 42 of the motor drive controller 31 according tothe second exemplary embodiment, and the integral controller 230 of themotor drive controller 210 according to the third exemplary embodiment.

Also, the motor drive controller 410 includes a first multiplier 451that multiplies, by a factor, the actual current detected by the motorcurrent detector 33, and a second multiplier 452 that multiplies, by afactor, the integral value obtained by integrating the value of theactual current detected by the motor current detector 33.

The first multiplier 451 multiplies, by “Kp×(α−1),” the actual currentdetected by the motor current detector 33, and outputs themultiplication result, as is the case with the multiplier 56 of themotor drive controller 31 according to the second exemplary embodiment.Also, the second multiplier 452 multiplies, by “Ki×(γ−1),” the integralvalue obtained by integrating the value of the actual current detectedby the motor current detector 33, and outputs the multiplication result,as is the case with the multiplier 250 of the motor drive controller 210according to the third exemplary embodiment.

Also, the motor drive controller 410 includes an adder 441 as an exampleof an addition unit that adds together the output value from theproportional controller 42, the output value from the integralcontroller 230, the output value from the first multiplier 451 and theoutput value from the second multiplier 452 and outputs the additionresult. The output value from the adder 441 is a base of a command valueto the electric motor 110, and the PWM signal generator 60 generates thePWM signal 60 a on the basis of the output value from the adder 441 andoutputs the generated PWM signal 60 a to the motor drive unit 32.

In the motor drive controller 410 according to the fifth exemplaryembodiment configured as described above, the first multiplier 451 andthe second multiplier 452 function as an adjusting unit that make anadjustment so that the denominator of the transfer function would remainconstant regardless of the values of the correction factors α and γ,when the target current set by the target current calculator 20 is takenas an input and the actual current actually supplied to the electricmotor 110 is taken as an output. This is proved by Equation (104) givenbelow.

FIG. 22 is a simple block diagram of the controller 400 according to thefifth exemplary embodiment. In FIG. 22, as in the case of FIG. 15, IT(s)represents the Laplace transform of the output value IT from the targetcurrent calculator 20, Im(s) represents the Laplace transform of theoutput value Im from the motor current detector 33, and P(s) representsin simple form the transfer function of the PWM signal generator 60, themotor drive unit 32 and the electric motor 110.

In this instance, a transfer function D(s) from IT(s) to Im(s) isexpressed by Equation (104).

$\begin{matrix}{{D(s)} = \frac{{\alpha \; s} + {\gamma \left( {{Ki}/{Kp}} \right)}}{{\left( {1 + {1/\left( {{P(s)} \times {Kp}} \right)}} \right)s} + \left( {{Ki}/{Kp}} \right)}} & (104)\end{matrix}$

As expressed by Equation (104), a denominator of the transfer functionD(s) is not affected by the correction factors α and γ.

When an input is IT(s) and an output is Im(s), a numerator of thetransfer function indicates the responsiveness of the actual current tothe target current. Thus, as can be seen from Equations (102) and (104),the motor drive controller 410 according to the fifth exemplaryembodiment enhances the effect of the proportional action performed bythe proportional action element 421 in the correction unit 422 and theeffect of the integral action performed by the integral action element231 in the correction unit 232, and correspondingly improves theresponsiveness, as compared to the comparative system.

Also, a denominator of the transfer function indicates the stability ofthe system, and as can be seen from Equations (102) and (104), thedenominator of D(s) is the same as that of G(s). Thus, the motor drivecontroller 410 according to the fifth exemplary embodiment ensures thesame stability as the comparative system, regardless of the values ofthe correction factors α and γ.

Therefore, as is the case with the motor drive controller 410 accordingto the fifth exemplary embodiment, the correction unit 422 that enhancesthe effect of the proportional action performed by the proportionalaction element 421 is provided in a part of the proportional controller42, the correction unit 232 that enhances the effect of the integralaction performed by the integral action element 231 is provided in apart of the integral controller 230, and also, the adder 441 adds thevalue obtained by multiplying Im by “Kp×(α−1),” and the value obtainedby a multiplying, by “Ki×(γ−1),” a value obtained by integrating Im, tothe output values from the proportional controller 42 and the integralcontroller 230, and thereby, an improvement in the responsiveness isachievable without affecting the stability of the system. Thereby, animprovement in steering feel is achievable without affecting thestability of the electric power steering apparatus 100. Also, at leastany one of the correction factors α and γ may be changed in accordancewith at least any one of the vehicle speed and the steering torquethereby to perform fine control.

If the existing system is the comparative system, the system may bemodified as is the case with the motor drive controller 410 according tothe fifth exemplary embodiment. Thereby, the same method as the existingsystem is used to set the proportional gain Kp and the integral gain Kiand thereby ensure the stability of the system, and then, the correctionfactors α and γ are adjusted to improve the responsiveness of the actualcurrent to the target current.

Sixth Exemplary Embodiment

FIG. 23 is a diagram showing an outline configuration of an electricpower steering apparatus 100 according to the sixth exemplaryembodiment.

The electric power steering apparatus 100 (hereinafter sometimes calledmerely the “steering apparatus 100”) acts as the steering apparatus forchanging the direction of travel of a vehicle into an any direction,and, in the sixth exemplary embodiment, exemplifies a configuration asapplied to an automobile.

The steering apparatus 100 includes a steering wheel 101 in the form ofwheel which a driver operates, and a steering shaft 102 providedintegrally with the steering wheel 101. The steering shaft 102 and anupper connecting shaft 103 are connected together via a universalcoupling 103 a, and the upper connecting shaft 103 and a lowerconnecting shaft 108 (as an example of a first rotary shaft) areconnected together via a universal coupling 103 b.

Also, the steering apparatus 100 includes tie rods 104 connectedrespectively to right and left front wheels 150 as a wheel to be turned,and a rack shaft 105 connected to the tie rods 104. Also, the steeringapparatus 100 includes a pinion 106 a that forms a rack-and-pinionmechanism in conjunction with rack teeth 105 a formed in the rack shaft105. The pinion 106 a is formed at a lower end portion of a pinion shaft106 (as an example of a second rotary shaft).

Also, the steering apparatus 100 includes a steering gear box 107 inwhich the pinion shaft 106 is housed. In the steering gear box 107, thepinion shaft 106 is connected to the lower connecting shaft 108 via atorsion bar (not shown in the figure). In addition, provided in thesteering gear box 107 is a torque sensor 109 as an example of a steeringtorque detector that detects steering torque of the steering wheel 101on the basis of a relative angle between the lower connecting shaft 108and the pinion shaft 106.

Also, the steering apparatus 100 includes an electric motor 110supported on the steering gear box 107, and a reduction gear mechanism111 that reduces drive power of the electric motor 110 and transfers thereduced drive power to the pinion shaft 106.

Also, the steering apparatus 100 includes a motor current detector 33(see FIG. 26) as an example of a current detector that detects themagnitude and direction of an actual current actually passing throughthe electric motor 110, and a motor voltage detector 160 that detects aterminal-to-terminal voltage of the electric motor 110.

The steering apparatus 100 includes a control device 10 that controlsactuation of the electric motor 110. Inputted to the control device 10are an output value from the above-mentioned torque sensor 109, anoutput value from a vehicle speed sensor 170 that detects the vehiclespeed of the automobile, an output value from the motor current detector33, and an output value from the motor voltage detector 160.

In the electric power steering apparatus 100 configured as describedabove, the steering torque applied to the steering wheel 101 is detectedby the torque sensor 109, the electric motor 110 is driven in accordancewith the detected torque, and torque produced by the electric motor 110is transmitted to the pinion shaft 106. Thereby, the torque produced bythe electric motor 110 assists the application of driver's steeringforce to the steering wheel 101.

Next, a description will be given with regard to the control device 10.

The control device 10 is an arithmetic logic circuit formed of a CPU, aROM, a RAM, a backup RAM and the like.

FIG. 24 is a schematic configuration diagram of the control device 10 ofthe electric power steering apparatus 100.

The control device 10 receives a torque signal Td obtained through theconversion of the steering torque detected by the above-mentioned torquesensor 109 into an output signal, and a vehicle speed signal v obtainedthrough the conversion of the vehicle speed detected by the vehiclespeed sensor 170 into an output signal.

Also, the control device 10 receives a motor current signal Im obtainedthrough the conversion of the actual current detected by the motorcurrent detector 33 into an output signal, and a terminal-to-terminalvoltage signal Vm of the motor, obtained through the conversion of thevoltage detected by the motor voltage detector 160 into an outputsignal.

Incidentally, since the detected signals in analog form are receivedfrom the torque sensor 109 and the like, the control device 10 uses anA/D converter (not shown in the figure) to convert the analog signalsinto digital signals and captures the digital signals in the CPU.

The control device 10 includes a target current calculator 20 thatcalculates target assist torque on the basis of the torque signal Td andcalculates a target current required for the electric motor 110 tosupply the target assist torque, and a controller 30 that performsfeedback control or the like on the basis of the target currentcalculated by the target current calculator 20.

Next, a detailed description will be given with regard to the targetcurrent calculator 20. FIG. 25 is a schematic configuration diagram ofthe target current calculator 20.

The target current calculator 20 includes a base current calculator 21that calculates a base current for use as a reference for setting thetarget current, and an inertia compensation current calculator 22 thatcalculates a current to cancel out the moment of inertia of the electricmotor 110. Also, the target current calculator 20 includes a dampercompensation current calculator 23 that calculates a current to limitmotor revolutions, and a motor revolution speed estimation unit 24 thatestimates a revolution speed signal Nm of the electric motor 110 on thebasis of the motor current signal Im and the terminal-to-terminalvoltage signal Vm of the motor.

Also, the target current calculator 20 includes a final target currentdetermination unit 25 that determines the final target current on thebasis of the outputs from the base current calculator 21, the inertiacompensation current calculator 22, the damper compensation currentcalculator 23, and so on.

Further, the target current calculator 20 includes a phase compensator26 that provides a phase compensation of the steering torque detected bythe torque sensor 109, and a resonance compensator 27 that provides aresonance compensation to eliminate resonance frequency components ofthe steering torque subjected to the phase compensation by the phasecompensator 26.

The phase compensator 26 performs filtering processing for the phasecompensation on the torque signal Td as the output value from the torquesensor 109, and outputs the torque signal Ts obtained through theprocessing. The resonance compensator 27 eliminates resonance frequencycomponents of the torque signal Ts, and outputs a torque signal Tpobtained through the elimination of the resonance frequency components.A detailed description will be given later with regard to the resonancecompensator 27.

The base current calculator 21 calculates the base current on the basisof the steering torque detected by the torque sensor 109 and the vehiclespeed detected by the vehicle speed sensor 170. More specifically, thebase current calculator 21 calculates the base current on the basis ofthe torque signal Tp as the output value from the resonance compensator27 and the vehicle speed signal v from the vehicle speed sensor 170, andoutputs the base current signal Ims containing information on the basecurrent. Incidentally, the calculation of the base current by the basecurrent calculator 21 is accomplished by, for example, substituting thetorque signal Tp and the vehicle speed signal v into a map showing thecorrespondence between a combination of the torque signal Tp and thevehicle speed signal v and the base current, which has previously beencreated on the basis of an empirical rule and been stored in the ROM.

The inertia compensation current calculator 22 calculates an inertiacompensation current to cancel out the moment of inertia of the electricmotor 110 and a system, on the basis of the torque signal Td and thevehicle speed signal v, and outputs an inertia compensation currentsignal Is containing information on the inertia compensation current.Incidentally, the calculation of the inertia compensation current by theinertia compensation current calculator 22 is accomplished by, forexample, substituting the torque signal Td and the vehicle speed signalv into a map showing the correspondence between a combination of thetorque signal Td and the vehicle speed signal v and the inertiacompensation current, which has previously been created on the basis ofan empirical rule and been stored in the ROM.

The damper compensation current calculator 23 calculates the dampercompensation current to limit the revolutions of the electric motor 110,on the basis of the torque signal Td, the vehicle speed signal v, andthe revolution speed signal Nm of the electric motor 110, and outputsthe damper compensation current signal Id containing information on thedamper compensation current. Incidentally, the calculation of the dampercompensation current by the damper compensation current calculator 23 isaccomplished by, for example, substituting the torque signal Td, thevehicle speed signal v and the revolution speed signal Nm into a mapshowing the correspondence between a combination of the torque signalTd, the vehicle speed signal v and the revolution speed signal Nm andthe damper compensation current, which has previously been created onthe basis of an empirical rule and been stored in the ROM.

The motor revolution speed estimation unit 24 estimates the revolutionspeed of the electric motor 110 on the basis of the actual currentdetected by the motor current detector 33 and the voltage detected bythe motor voltage detector 160.

The final target current determination unit 25 determines the finaltarget current on the basis of the base current signal Ims outputted bythe base current calculator 21, the inertia compensation current signalIs outputted by the inertia compensation current calculator 22, and thedamper compensation current signal Id outputted by the dampercompensation current calculator 23, and outputs a target current signalIT containing information on the final target current. The calculationof the final target current by the final target current determinationunit 25 is accomplished by, for example, substituting a compensationcurrent obtained by adding the inertia compensation current to the basecurrent and also subtracting the damper compensation current from theadded result, into a map showing the correspondence between thecompensation current and the final target current, which has previouslybeen created on the basis of an empirical rule and been stored in theROM.

As described above, the target current calculator 20 functions as anexample of a target current setting unit that sets the target current tobe supplied to the electric motor 110, on the basis of the steeringtorque detected by the torque sensor 109.

Next, a detailed description will be given with regard to the controller30. FIG. 26 is a schematic configuration diagram of the controller 30.

The controller 30 includes a motor drive controller 31 that controls theactuation of the electric motor 110, a motor drive unit 32 that drivesthe electric motor 110, and the motor current detector 33 that detectsthe actual current actually passing through the electric motor 110.

The motor drive controller 31 includes a feedback (F/B) controller 40that performs feedback control on the basis of a deviation between thetarget current calculated by the target current calculator 20 and theactual current detected by the motor current detector 33 and supplied tothe electric motor 110 and a pulse width modulation (PWM) signalgenerator 60 that generates a PWM signal to provide PWM drive to theelectric motor 110.

The feedback controller 40 includes a deviation calculator 41 thatdetermines the deviation between the target current calculated by thetarget current calculator 20 and the actual current detected by themotor current detector 33, and a feedback (F/B) processor 45 thatperforms feedback processing so that the deviation would become zero.

The deviation calculator 41 outputs, as a deviation signal 41 a, thevalue of the deviation between the output value IT from the targetcurrent calculator 20 and the output value Im from the motor currentdetector 33.

The feedback (F/B) processor 45 serves to perform feedback control sothat the actual current would coincide with the target current, andgenerates and outputs a feedback processing signal 42 a by, for example,using a proportional element to perform proportional processing on theinputted deviation signal 41 a and output the proportional-processedsignal; using an integral element to perform integral processing on theinputted deviation signal 41 a and output the integral-processed signal;and using an add operation unit to add these processed signals together.

The PWM signal generator 60 generates a PWM signal 60 a on the basis ofan output value from the feedback (F/B) controller 40, and outputs thegenerated PWM signal 60 a.

The motor drive unit 32 includes a motor drive circuit 70 formed of fourfield-effect transistors for electric power connected in theconfiguration of an H type bridge circuit, and a gate drive circuit unit80 that drives gates of two field-effect transistors selected from amongthe four field effect transistors thereby to bring the two selectedfield-effect transistors into switching operation. The gate drivecircuit unit 80 selects two field-effect transistors in accordance withthe steering direction of the steering wheel 101, on the basis of adrive control signal (the PWM signal 60 a) outputted by the PWM signalgenerator 60, and brings the two selected field-effect transistors intoswitching operation.

The motor current detector 33 detects the value of a motor current (oran armature current) passing through the electric motor 110, from avoltage between both ends of a shunt resistor 71 connected in serieswith the motor drive circuit 70, and outputs the motor current signalIm.

Next, a description will be given with regard to the resonancecompensator 27.

The steering apparatus 100 is a control system including the torsion bar(not shown in the figure) for use in steering torque detection as thespring element, and the electric motor 110, the pinion shaft 106 and therack shaft 105 as the inertial elements. Thus, for instance, when thefeedback processor 45 includes the proportional element and the integralelement, if the values of the proportional gain and the integral gain ofthe feedback processor 45 are increased in order to heighten theresponsiveness of the system taken as a whole, the system is likely tobecome unstable (or vibrated) in the vicinity of the resonance frequencyof the control system.

The resonance compensator 27 is provided in order to eliminate orsuppress a peak in a resonance frequency band of the control system.

A transfer function H₁(s) indicating characteristics of the resonancecompensator 27 defines a transfer function G₁ (s) indicatingcharacteristics of the control system of the steering apparatus 100 inthe following manner. Specifically, a numerator of H₁(s) has the sameelement as a denominator of G₁(s), and the degree of a denominator ofH₁(s) is a second-order degree equal to or higher than the numerator inorder to ensure feasibility of the resonance compensator 27. In otherwords, the resonance compensator 27 defines its transfer function so asto have a filtering function having an antiresonant element of thecontrol system and a low-pass filtering function.

Firstly, discussion will be made with regard to the transfer functionG₁(s).

The reduction gear mechanism 111 of the steering apparatus 100 is formedof a worm wheel (not shown in the figure) mounted to the pinion shaft106, and a worm gear (not shown in the figure) mounted to an outputshaft of the electric motor 110.

In such an instance, when τ_(m)(N·m) represents the torque of theelectric motor 110, θ₁ (rad) represents the angle of rotation, τ₁ (N·m)represents the torque of the worm gear, and J₁ (kg·m²) represents theinertia of the motor shaft, an equation of motion of the electric motor110 is expressed by Equation (1) below.

τ_(m) =J ₁·{umlaut over (θ)}₁+τ₁  (1)

Here, the inertia J₁ of the motor shaft is represented asJ₁=J_(m)+J_(T1), where J_(m) (kg·m²) represents the inertia of themotor; and J_(T1) (kg·m²) represents the inertia of the worm gear.

Also, when θ₂ (rad) represents the angle of rotation of the pinion shaft106, τ₂ (N·m) represents the torque of the worm wheel, τ₃ (N·m)represents the torque of the pinion 106 a, J₂ (kg·m²) represents theinertia of the pinion shaft, and k_(tb) (N·m/rad) represents a springconstant of the torsion bar, an equation of motion of the pinion shaft106 is expressed by Equation (2) below.

τ₂ =J ₂·{umlaut over (θ)}₂ +k _(tb)·θ₂+τ₃  (2)

Here, the inertia J₂ of the pinion shaft is represented asJ₂=J_(T2)+J_(T3), where J_(T2) (kg·m²) represents the inertia of theworm wheel; and J_(T3) (kg·m²) represents the inertia of the pinion 106a.

Also, when x (m) represents displacement of the rack shaft 105, m (kg)represents mass, and r (m) represents a radius of rotation of the pinion106 a, an equation of motion of the rack shaft 105 is expressed byEquation (3) below.

$\begin{matrix}{\frac{\tau_{3}}{r} = {m \cdot \overset{¨}{x}}} & (3)\end{matrix}$

Here, the displacement x of the rack shaft 105 is expressed by Equation(4) below.

x=r·θ ₂  (4)

Also, a worm speed reduction ratio γ₁ and a rack-and-pinion ratio γ₂(m/rev) are expressed by Equations (5) and (6) below, respectively.

γ₁=θ₁/θ₂=τ₂/τ₁  (5)

γ₂=2·π·r  (6)

Equation (7) is derived from Equations (3), (4) and (6).

$\begin{matrix}{\tau_{3} = {{m \cdot \overset{¨}{x} \cdot r} = {{m \cdot \left( \frac{Y_{2}}{2\pi} \right) \cdot {\overset{¨}{\theta}}_{2} \cdot \frac{Y_{2}}{2\pi}} = {\frac{m \cdot Y_{2}^{2}}{4\pi^{2}} \cdot {\overset{¨}{\theta}}_{2}}}}} & (7)\end{matrix}$

Equation (8) is derived from Equations (1), (2) and (5).

$\begin{matrix}\begin{matrix}{\tau_{m} = {{J_{1} \cdot Y_{1} \cdot {\overset{¨}{\theta}}_{2}} + {\frac{1}{Y_{1}}\tau_{2}}}} \\{= {{J_{1} \cdot Y_{1} \cdot {\overset{¨}{\theta}}_{2}} + {\frac{1}{Y_{1}} \cdot \left( {{J_{2} \cdot {\overset{¨}{\theta}}_{2}} + {k_{tb} \cdot \theta_{2}} + \tau_{3}} \right)}}}\end{matrix} & (8)\end{matrix}$

Substituting Equation (8) into Equation (7) to organize the equationsleads to Equation (9).

$\begin{matrix}{{Y_{1} \cdot \tau_{m}} = {{\left( {{Y_{1}^{2} \cdot J_{1}} + J_{2} + \frac{m \cdot Y_{2}^{2}}{4\pi^{2\;}}} \right) \cdot {\overset{¨}{\theta}}_{2}} + {k_{tb} \cdot \theta_{2}}}} & (9)\end{matrix}$

Laplace transform of Equation (9) to organize the equation leads toEquation (10).

$\begin{matrix}{{\Theta_{2}(s)} = {\frac{1}{{\left( {{\gamma_{1} \cdot J_{1}} + \frac{J_{2}}{\gamma_{1}} + \frac{m \cdot Y_{2}^{2}}{4{\pi^{2} \cdot \gamma_{1}}}} \right) \cdot s^{2}} + \frac{k_{tb}}{\gamma_{1}}} \cdot {T_{m}(s)}}} & (10)\end{matrix}$

Incidentally, s denotes an operator for the Laplace transform. Also,T_(m)(s) represents the Laplace transform of the torque T_(m) of theelectric motor 110; and Θ₂ (s) represents the Laplace transform of theangle θ₂ of rotation of the pinion shaft 106.

From Equation (10), the above transfer function G₁ (s) is expressed byEquation (11).

$\begin{matrix}{{G_{1}(s)} = \frac{1}{{\left( {{\gamma_{1} \cdot J_{1`}} + \frac{J_{2}}{\gamma_{1}} + \frac{m \cdot Y_{2}^{2}}{4{\pi^{2} \cdot \gamma_{1}}}} \right) \cdot s^{2}} + \frac{k_{tb}}{\gamma_{1}}}} & (11)\end{matrix}$

Also, from Equation (11), a resonance angular frequency ω₁ is expressedby Equation (12).

$\begin{matrix}{\omega_{1} = \sqrt{\frac{k_{tb}}{{Y_{1}^{2} \cdot J_{1}} + J_{2} + \frac{m \cdot Y_{2}^{2}}{4\pi^{2}}}}} & (12)\end{matrix}$

Therefore, the transfer function H₁ (s) indicating the characteristicsof the resonance compensator 27 is represented as “a₁(2πf_(c1))·(2πf_(c2))/((s+2πf_(c1))+2πf_(c2)).”Incidentally, a₁ is avalue expressed by Equation (13).

$\begin{matrix}{a_{1} = \frac{{\left( {{4{\pi^{2} \cdot Y_{1}^{2} \cdot J_{1}}} + {4{\pi^{2} \cdot J_{2}}} + {m \cdot Y_{2}^{2}}} \right)s^{2}} + {4{\pi^{2} \cdot k_{tb}}}}{4{\pi^{2} \cdot \gamma_{1}}}} & (13)\end{matrix}$

Then, the degree of the denominator of the transfer function H₁(s) isset to the second-order degree, which is the lowest one of the degreescapable of ensuring the feasibility of the resonance compensator 27, andlow-pass filters (LPFs) are provided in two stages. f_(c1) and f_(c2)represent cut-off frequencies of the LPFs.

FIGS. 27A and 27B are bode diagrams showing a comparison of frequencycharacteristics of the control system between the presence of theresonance compensator 27 and the absence of the resonance compensator27.

FIGS. 27A and 27B are the diagrams showing the results of simulations(or numerical experiments) performed on the control system in which theoperation of the steering wheel 101 is an input and the angle ofrotation of the pinion 106 a is an output, and FIGS. 27A and 27B are again characteristic plot and a phase characteristic plot, respectively.In FIGS. 27A and 27B, a solid line indicates the results in the case ofthe presence of the resonance compensator 27, and a dashed lineindicates the results in the case of the absence of the resonancecompensator 27. Incidentally, the denominator of the transfer functionH₁(s) is set at a value equivalent to a two-stage construction of thelow-pass filters each having a cut-off frequency of 100 Hz.

As shown in FIG. 27A, the provision of the resonance compensator 27permits reducing or canceling out the peak of the resonance frequencycomponent. Also, as shown in FIG. 27B, the presence of the resonancecompensator 27 achieves a great improvement in phase delay, as comparedto the absence of the resonance compensator 27. From these results, itcan be seen that the provision of the resonance compensator 27 achievesan improvement in the stability. Also, the provision of the resonancecompensator 27 allows an increase in a gain-crossover frequency andhence an improvement in the responsiveness.

Incidentally, the description has been given with regard to the electricpower steering apparatus 100 of a pinion type; however, it is preferablethat an electric power steering apparatus of a column type likewiseinclude a resonance compensator such that the transfer function is H₁(s) mentioned above.

Seventh Exemplary Embodiment

FIG. 28 is a diagram showing an outline configuration of an electricpower steering apparatus 200 according to the seventh exemplaryembodiment. Hereinafter, a description will be given with regard to thedifference between the seventh exemplary embodiment and the sixthexemplary embodiment. The same components are denoted by the samereference numerals, and the detailed description thereof will beomitted.

The electric power steering apparatus 200 (hereinafter sometimes calledmerely the “steering apparatus 200”) according to the seventh exemplaryembodiment is what is called an electric power steering apparatus of arack assist type, and applies torque produced by an electric motor 201to the rack shaft 105.

Specifically, the electric motor 201 according to the seventh exemplaryembodiment includes a stator (not shown in the figure) mounted to ahousing (not shown in the figure), and a rotor (not shown in the figure)mounted rotatably to the housing about the axis of the rack shaft 105 asthe axis of rotation and also unmovably in an axial direction of therack shaft 105. The rotor engages a ball screw nut through an elasticbody, and the rotor produces assist force that effects rotation of theball screw nut through the elastic body and thereby effects an axialmovement of the rack shaft 105. Also, an output from the rotor iscontrolled by a controller 210 according to the seventh exemplaryembodiment. Thereby, under control of the controller 210, the torqueproduced by the electric motor 201 is transmitted to the rack shaft 105and thereby assists the application of the driver's steering force tothe steering wheel 101.

FIG. 29 is a schematic configuration diagram of the controller 210according to the seventh exemplary embodiment.

As is the case with the control device 10 according to the sixthexemplary embodiment, the controller 210 includes a target currentcalculator 220 that calculates the target assist torque on the basis ofthe torque signal Td and calculates the target current required for theelectric motor 201 to supply the target assist torque, and a controller235 that performs feedback control or the like on the basis of thetarget current calculated by the target current calculator 220.

The controller 235 has the same function and configuration as thecontroller 30 of the control device 10 according to the sixth exemplaryembodiment. The target current calculator 220 is different from thetarget current calculator 20 according to the sixth exemplary embodimentin that the target current calculator 220 has a resonance compensator271 different from the resonance compensator 27, and in other respects,the target current calculator 220 has the same function andconfiguration as the target current calculator 20 according to the sixthexemplary embodiment.

The resonance compensator 271 is provided in order to eliminate orsuppress the peak in the vicinity of the resonance frequency of thecontrol system of the electric power steering apparatus 200 according tothe seventh exemplary embodiment, the control system including thetorsion bar (not shown in the figure) as the spring element, and theelectric motor 201, the pinion shaft 106 and the rack shaft 105 as theinertial elements. Therefore, a transfer function H₂ (s) indicatingcharacteristics of the resonance compensator 271 defines a transferfunction G₂ (s) indicating characteristics of the control system of thesteering apparatus 200 in the following manner. Specifically, anumerator of H₂ (s) has the same element as a denominator of G₂ (s), andthe degree of a denominator of H₂ (s) is the second-order degree, whichis the lowest one of the degrees capable of ensuring the feasibility ofthe resonance compensator 271, and LPFs are provided in two stages.

Firstly, discussion will be made with regard to the transfer function G₂(s).

When τ_(m) (N·m) represents the torque of the electric motor 201, θ₁(rad) represents the angle of rotation, τ₁ (N·m) represents the torqueof the ball screw nut, and J₁ (kg·m²) represents the inertia of themotor shaft, an equation of motion of the electric motor 201 isexpressed by Equation (14) below.

τ_(m) =J ₁·{umlaut over (θ)}₁+τ₁  (14)

Here, the inertia J₁ of the motor shaft is represented as J₁=J_(m),+J_(T1), where J_(m), (kg·m²) represents the inertia of the motor; andJ_(T1) (kg·m²) represents the inertia of the ball screw nut.

Also, when τ₂ (N·m) represents the torque of the pinion shaft 106, θ₂(rad) represents the angle of rotation, J₂ (kg·m²) represents theinertia of the pinion shaft, and k_(tb) (N·m/rad) represents the springconstant of the torsion bar, an equation of motion of the pinion shaft106 is expressed by Equation (15) below.

τ=J ₂·{umlaut over (θ)}₂ +k _(tb)·θ₂  (15)

Also, when x (m) represents the displacement of the rack shaft 105, m(kg) represents the mass, r₁ represents a radius of rotation of the ballscrew nut, and r₂ (m) represents the radius of rotation of the pinion106 a, an equation of motion of the rack shaft 105 is expressed byEquation (16) below.

$\begin{matrix}{\frac{\tau_{1}}{r_{1}} = {{m \cdot \overset{¨}{x}} + \frac{\tau_{2}}{r_{2}}}} & (16)\end{matrix}$

Here, the displacement x of the rack shaft 105 is expressed by Equation(17) or (18).

x=r ₁·θ₁=γ₁/(2·π)×θ₁  (17)

x=r ₂·θ₂=γ₂/(2·π)×θ₂  (18)

Incidentally, γ₁ is a ratio (m/rev) indicating a distance traveled bythe rack shaft 105 during one revolution of the ball screw nut, and isrepresented as γ₁=2·πr₁. Also, γ₂ is a ratio (m/rev) indicating adistance traveled by the rack shaft 105 during one revolution of thepinion shaft 106, and is represented as γ₂=2·πr₂.

Also, Equation (19) is derived from Equations (17) and (18).

θ₁=γ₂/γ₁×θ₂  (19)

Also, Equation (20) is derived from Equations (16) and (17).

$\begin{matrix}{r_{1} = {{\frac{r_{\overset{.}{1}}Y_{\overset{.}{2}}m}{2 \cdot \pi} \cdot \overset{¨}{\theta_{2}}} + {\frac{r_{1}}{r_{2}} \cdot \tau_{2}}}} & (20)\end{matrix}$

Then, Equation (21) is derived from Equations (14) to (20).

4π²·γ₁·γ₂·τ_(m)=(4π²·γ₂ ² ·J ₁+γ₁ ²+γ₁ ²·γ₂ ² ·m+4π²·γ₁ ² ·J ₂)·{umlautover (θ)}₂+4π²·γ₁ ² ·k _(tb)·θ₂  (21)

Laplace transform of Equation (21) to organize the equation leads toEquation (22).

$\begin{matrix}{{\Theta_{2}(s)} = {\frac{1}{{\left( {{\frac{\mathrm{\Upsilon}_{2}}{\gamma_{1}} \cdot J_{1}} + {\frac{\mathrm{\Upsilon}_{1}}{\gamma_{2}} \cdot J_{2}} + \frac{{m \cdot \mathrm{\Upsilon}_{\overset{.}{1}}}\mathrm{\Upsilon}_{2}}{4\pi^{2}}} \right) \cdot s^{2}} + {\frac{\mathrm{\Upsilon}_{1}}{\gamma_{2}} \cdot k_{tb}}} \cdot {T_{m}(s)}}} & (22)\end{matrix}$

Incidentally, s denotes an operator for the Laplace transform. Also,T_(m)(s) represents the Laplace transform of the torque τ_(m) of theelectric motor 201; and Θ₂ (s) represents the Laplace transform of theangle θ₂ of rotation of the pinion shaft 106.

From Equation (22), the above transfer function G₂(s) is expressed byEquation (23).

$\begin{matrix}{{G_{2}(s)} = \frac{1}{{\left( {{\frac{\mathrm{\Upsilon}_{2}}{\gamma_{1}} \cdot J_{1}} + {\frac{\mathrm{\Upsilon}_{1}}{\gamma_{2}} \cdot J_{2}} + \frac{{m \cdot \mathrm{\Upsilon}_{\overset{.}{1}}}\mathrm{\Upsilon}_{2}}{4\pi^{2}}} \right) \cdot s^{2}} + {\frac{\mathrm{\Upsilon}_{1}}{\gamma_{2}} \cdot k_{tb}}}} & (23)\end{matrix}$

Also, from Equation (23), a resonance angular frequency ω₂ is expressedby Equation (24).

$\begin{matrix}{\omega_{2} = \sqrt{\frac{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}k_{tb}}{{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}J_{1}} + {\mathrm{\Upsilon}_{\overset{.}{1}}^{2}\mathrm{\Upsilon}_{\overset{.}{2}}^{2}m} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}J_{2}}}}} & (24)\end{matrix}$

Therefore, the degree of the denominator of the transfer function H₂ (s)indicating the characteristics of the resonance compensator 271 is setto the second-order degree, which is the lowest one of the degreescapable of ensuring the feasibility of the resonance compensator 271,and LPFs are provided in two stages. Specifically, the transfer functionH₂(s) is represented as“H₂(s)=a₂·((2πf_(c1))·(2πf_(c2)))/((s+2πf_(c1))×(s+2πf_(c2))).”Incidentally, a_(z) is a value expressed by Equation (25). Also, f_(c1)and f_(c2) represent the cut-off frequencies of the LPFs.

$\begin{matrix}{a_{2} = \frac{{\begin{pmatrix}{{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}J_{1}} +} \\{{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}J_{2}} + {{m \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}\mathrm{\Upsilon}_{2}^{2}}}\end{pmatrix} \cdot s^{2}} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}k_{tb}}}{4{\pi^{2} \cdot \gamma_{\overset{.}{1}}}\gamma_{2}}} & (25)\end{matrix}$

The provision of the resonance compensator 271 in which the transferfunction is H₂ (s) permits reducing or canceling out the peak of theresonance frequency component also in the power steering apparatus 200according to the seventh exemplary embodiment. Also, the presence of theresonance compensator 271 allows a great improvement in the phase delay,as compared to the absence of the resonance compensator 271. Thereby,the provision of the resonance compensator 271 allows an improvement inthe stability. Also, the provision of the resonance compensator 271allows an increase in the gain-crossover frequency and hence animprovement in the responsiveness.

Eighth Exemplary Embodiment

FIG. 30 is a diagram showing an outline configuration of an electricpower steering apparatus 300 according to an eighth exemplaryembodiment. Hereinafter, a description will be given with regard to thedifference between the eighth exemplary embodiment and the sixthexemplary embodiment. The same components are denoted by the samereference numerals, and the detailed description thereof will beomitted.

The electric power steering apparatus 300 (hereinafter sometimes calledmerely the “steering apparatus 300”) according to the eighth exemplaryembodiment is what is called an electric power steering apparatus of adouble-pinion type, and applies torque produced by an electric motor 301to the rack shaft 105 through a pinion 302 a of a second pinion shaft302. The second pinion shaft 302 is a member provided aside from thepinion shaft 106 connected to the steering wheel 101 via the torsionbar, as shown in FIG. 30.

As mentioned above, the steering apparatus 300 according to the eighthexemplary embodiment includes the second pinion shaft 302. A worm wheel303 mounted to the second pinion shaft 302 is connected to a worm gear(not shown in the figure) mounted to an output shaft of the electricmotor 301. An output from the electric motor 301 is controlled by acontroller 310 according to the eighth exemplary embodiment. Thereby,under control of the controller 310, the torque produced by the electricmotor 301 is transmitted to the rack shaft 105 and thereby assists theapplication of the driver's steering force to the steering wheel 101.

FIG. 31 is a schematic configuration diagram of the controller 310according to the eighth exemplary embodiment.

As is the case with the control device 10 according to the sixthexemplary embodiment, the controller 310 includes a target currentcalculator 320 that calculates the target assist torque on the basis ofthe torque signal Td and calculates the target current required for theelectric motor 301 to supply the target assist torque, and a controller330 that performs feedback control or the like on the basis of thetarget current calculated by the target current calculator 320.

The controller 330 has the same function and configuration as thecontroller 30 of the control device 10 according to the sixth exemplaryembodiment. The target current calculator 320 is different from thetarget current calculator 20 according to the sixth exemplary embodimentin that the target current calculator 320 has a resonance compensator272 different from the resonance compensator 27, and in other respects,the target current calculator 320 has the same function andconfiguration as the target current calculator 20 according to the sixthexemplary embodiment.

The resonance compensator 272 is provided in order to eliminate orsuppress the peak in the vicinity of the resonance frequency of thecontrol system of the steering apparatus 300 according to the eighthexemplary embodiment, the control system including the torsion bar (notshown in the figure) as the spring element, and the electric motor 301,the pinion shaft 106, the second pinion shaft 302 and the rack shaft 105as the inertial elements. Therefore, a transfer function H₃(s)indicating characteristics of the resonance compensator 272 defines atransfer function G₃(s) indicating characteristics of the mechanicalvibration system of the steering apparatus 300 in the following manner.Specifically, a numerator of H₃(s) has the same element as a denominatorof G₃(s), and the degree of a denominator of H₃(s) is the second-orderdegree, which is the lowest one of the degrees capable of ensuring thefeasibility of the resonance compensator 272, and LPFs are provided intwo stages.

Firstly, discussion will be made with regard to the transfer functionG₃(s).

When τ_(r), (N·m) represents the torque of the electric motor 301, θ₁(rad) represents the angle of rotation, τ₁ (N·m) represents the torqueof the worm gear, and J₁ (kg·m²) represents the inertia of the motorshaft, an equation of motion of the electric motor 301 is expressed byEquation (26) below.

τ_(m) =J ₁·{umlaut over (θ)}₁+τ₁  (26)

Here, the inertia J₁ of the motor shaft is represented asJ₁=J_(m)+J_(T1), where J_(m) (kg·m²) represents the inertia of themotor; and J_(T1) (kg·m²) represents the inertia of the worm gear.

Also, when θ₂ (rad) represents the angle of rotation of the secondpinion shaft 302, τ₂ (N·m) represents the torque of the worm wheel 303,τ₃ (N·m) represents the torque of the pinion 302 a of the second pinionshaft 302, and J₂ (kg·m²) represents the inertia of the pinion shaft, anequation of motion of the second pinion shaft 302 is expressed byEquation (27) below.

τ₂ =J ₂·{umlaut over (θ)}₂+τ₃  (27)

Here, the inertia J₂ of the pinion shaft is represented asJ₂=J_(T2)+J_(T3), where J_(T2) (kg·m²) represents the inertia of theworm wheel; and J_(T3) (kg·m²) represents the inertia of the secondpinion shaft 302.

Also, when θ₃ (rad) represents the angle of rotation of the pinion shaft106, τ₄ (N·m) represents the torque of the pinion 106 a of the pinionshaft 106, J₃ (kg·m²) represents the inertia of the pinion shaft andk_(tb) (N·m/rad) represents the spring constant of the torsion bar, anequation of motion of the pinion shaft 106 is expressed by Equation (28)below.

τ₄ =J ₃·{umlaut over (θ)}₃ +k _(tb)·θ₃  (28)

Also, when x (m) represents the displacement of the rack shaft 105, m(kg) represents the mass, r₁ (m) represents a radius of rotation of thepinion 302 a of the second pinion shaft 302, and r₂ (m) represents theradius of rotation of the pinion 106 a of the pinion shaft 106, anequation of motion of the rack shaft 105 is expressed by Equation (29).

$\begin{matrix}{\frac{\tau_{3}}{r_{1}} = {{m \cdot \overset{¨}{x}} + \frac{\tau_{4}}{r_{2}}}} & (29)\end{matrix}$

Here, the displacement x of the rack shaft 105 is expressed by Equation(30).

x=r ₁·θ₂ =r ₂·θ₃  (30)

Also, when γ₁ represents a worm speed reduction ratio, γ₂ (m/rev)represents a ratio indicating a distance traveled by the rack shaft 105during one revolution of the second pinion shaft 302, and γ₃ (m/rev)represents a ratio indicating a distance traveled by the rack shaft 105during one revolution of the pinion shaft 106, γ₁, γ₂ and γ₃ areexpressed by Equations (31), (32) and (33), respectively.

γ₁=θ/θ₂=τ₂/τ₁  (31)

γ₂=2·r ₁  (32)

γ₃=2·π·r ₂  (33)

Equation (34) is derived from Equations (26), (30) and (31).

$\begin{matrix}{r_{m} = {{\gamma_{\overset{.}{1}}J_{\overset{.}{1}}{\frac{r_{2}}{r_{1}} \cdot \overset{¨}{\theta_{3}}}} + {\frac{1}{\gamma_{1}} \cdot \tau_{2}}}} & (34)\end{matrix}$

Also, Equation (35) is derived from Equations (27) and (30).

$\begin{matrix}{r_{2} = {{J_{\overset{.}{2}}{\frac{r_{2}}{r_{1}} \cdot \overset{¨}{\theta_{3}}}} + \tau_{3}}} & (35)\end{matrix}$

Therefore, Equation (36) is derived from Equations (34) and (35).

$\begin{matrix}{r_{m} = {{\gamma_{\overset{.}{1}}J_{\overset{.}{1}}{\frac{r_{2}}{r_{1}} \cdot \overset{¨}{\theta_{3}}}} + {\frac{1}{\gamma_{1}} \cdot \left( {{J_{\overset{.}{2}}{\frac{r_{2}}{r_{1}} \cdot \overset{¨}{\theta_{3}}}} + \tau_{3}} \right)}}} & (36)\end{matrix}$

Also, Equation (37) is derived from Equations (28), (29) and (30).

$\begin{matrix}{r_{3} = {{r_{\overset{.}{1}}r_{\overset{.}{2}}{m \cdot \overset{¨}{\theta_{3}}}} + {J_{\overset{.}{3}}{\frac{r_{1}}{r_{2}} \cdot \overset{¨}{\theta_{3}}}} + {\frac{r_{1}}{r_{2}} \cdot k_{tb} \cdot \theta_{3}}}} & (37)\end{matrix}$

Then, Equation (38) is derived from Equations (36) and (37).

r ₁ ·r ₂·γ₁·τ_(m)=(r ₂ ²·γ₁ ² ·J ₁+γ₁ ²·γ₂ ² ·m+r ₂ ² ·J ₂ +r ₁ ² ·J₃·){umlaut over (θ)}₃ +r ₁ ² ·k _(tb)·θ₃  (38)

Laplace transform of an equation obtained by substituting Equations (32)and (33) into Equation (38) to organize the equations leads to Equation(39).

$\begin{matrix}{{{\Theta_{3}(s)} = {\frac{1}{{\begin{pmatrix}{{\frac{\mathrm{\Upsilon}_{\overset{.}{1}}\mathrm{\Upsilon}_{3}}{\gamma_{2}} \cdot J_{1}} + {\frac{\mathrm{\Upsilon}_{3}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{2}} \cdot J_{2}} +} \\{\frac{{m \cdot \mathrm{\Upsilon}_{\overset{.}{2}}}\mathrm{\Upsilon}_{3}}{4{\pi^{2} \cdot \gamma_{1}}} + {\frac{\mathrm{\Upsilon}_{2}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{3}} \cdot J_{3}}}\end{pmatrix} \cdot s^{2}} + {\frac{\mathrm{\Upsilon}_{2}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{3}} \cdot k_{tb}}} \cdot {T_{m}(s)}}}\;} & (39)\end{matrix}$

Incidentally, s denotes an operator for the Laplace transform. Also,T_(m)(s) represents the Laplace transform of the torque τ_(m) of theelectric motor 301, and Θ₃(s) represents the Laplace transform of theangle θ₃ of rotation of the pinion shaft 106.

From Equation (39), the above transfer function G₃(s) is expressed byEquation (40).

$\begin{matrix}{{{G_{3}(s)} = \frac{1}{{\begin{pmatrix}{{\frac{\mathrm{\Upsilon}_{\overset{.}{1}}\mathrm{\Upsilon}_{3}}{\gamma_{2}} \cdot J_{1}} + {\frac{\mathrm{\Upsilon}_{3}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{2}} \cdot J_{2}} +} \\{\frac{{m \cdot \mathrm{\Upsilon}_{\overset{.}{2}}}\mathrm{\Upsilon}_{3}}{4{\pi^{2} \cdot \gamma_{1}}} + {\frac{\mathrm{\Upsilon}_{2}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{3}} \cdot J_{3}}}\end{pmatrix} \cdot s^{2}} + {\frac{\mathrm{\Upsilon}_{2}}{\mathrm{\Upsilon}_{\overset{.}{1}}\gamma_{3}} \cdot k_{tb}}}}\;} & (40)\end{matrix}$

Also, from Equation (40), a resonance angular frequency ω₃ is expressedby Equation (41).

$\begin{matrix}{\omega_{3} = \sqrt{\frac{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}k_{tb}}{{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}\mathrm{\Upsilon}_{\overset{.}{3}}^{2}J_{1}} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{3}}^{2}}J_{2}} + {\mathrm{\Upsilon}_{\overset{.}{2}}^{2}\mathrm{\Upsilon}_{\overset{.}{3}}^{2}m} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}J_{3}}}}} & (41)\end{matrix}$

Then, the degree of the denominator of the transfer function H₃ (S)indicating the characteristics of the resonance compensator 272 is setto the second-order degree, which is the lowest one of the degreescapable of ensuring the feasibility of the resonance compensator 272,and LPFs are provided in two stages. Specifically, the transfer functionH₃(s) is represented as“H₃(s)=a₃·((2πf_(c1))·(2πf_(c2)))/((s+2πf_(c1))·(s+2πf_(c2))).”Incidentally, a₃ is a value expressed by Equation (42). Also, f_(c1) andf_(c2) represent the cut-off frequencies of the LPFs.

$\begin{matrix}{a_{3} = \frac{{\begin{pmatrix}{{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}\mathrm{\Upsilon}_{\overset{.}{3}}^{2}J_{1}} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{3}}^{2}}J_{2}} + {\mathrm{\Upsilon}_{\overset{.}{2}}^{2}\mathrm{\Upsilon}_{\overset{.}{3}}^{2}m} +} \\{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}J_{3}}\end{pmatrix} \cdot s^{2}} + {4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{2}}^{2}}k_{tb}}}{4{\pi^{2} \cdot \gamma_{\overset{.}{1}}}\gamma_{\overset{.}{2}}\gamma_{3}}} & (42)\end{matrix}$

The provision of the resonance compensator 272 in which the transferfunction is H₃ (s) permits reducing or canceling out the peak of theresonance frequency component also in the power steering apparatus 300according to the eighth exemplary embodiment. Also, the presence of theresonance compensator 272 allows a great improvement in the phase delay,as compared to the absence of the resonance compensator 272. Thereby,the provision of the resonance compensator 272 allows an improvement inthe stability. Also, the provision of the resonance compensator 272allows an increase in the gain-crossover frequency and hence animprovement in the responsiveness.

Incidentally, it is preferable that a correction factor varyingaccording to the circumstances be used to correct the transfer functionsof the resonance compensators 27, 271 and 272 according to theabove-mentioned sixth to eighth exemplary embodiments.

Specifically, preferably, in the transfer functions“H_(n)(s)=a_(n)·((2πf_(c1))·(2πf_(c2)))/((s+2πf_(c1))·(s+2πf_(c2)))(where n=1, 2, 3)” of the resonance compensators 27, 271 and 272according to the above-mentioned sixth to eighth exemplary embodiments,the operator “s” for the Laplace transform in a_(n) is replaced by“α₁×s,” where α₁ represents the correction factor, and the correctedtransfer function is used as the transfer function of any one of theresonance compensators 27, 271 and 272.

For instance, the transfer function H₁(s) of the resonance compensator27 according to the sixth exemplary embodiment is corrected as expressedby Equation (43).

$\begin{matrix}{{{H_{1f}(s)} = \begin{matrix}{\frac{{\left( {{4{\pi^{2} \cdot \mathrm{\Upsilon}_{\overset{.}{1}}^{2}}J_{1}} + {4{\pi^{2} \cdot J_{2}}} + {m \cdot \mathrm{\Upsilon}_{2}^{2}}} \right) \cdot ({as})^{2}} + {4{\pi^{2} \cdot k_{tb}}}}{4{\pi^{2} \cdot \gamma_{1}}} \cdot} \\\frac{\left( {2{\pi \cdot f_{c\; 1}}} \right) \cdot \left( {2{\pi \cdot f_{c\; 2}}} \right)}{\left( {s + {2{\pi \cdot f_{c\; 1}}}} \right) \cdot \left( {s + {2{\pi \cdot f_{c\; 2}}}} \right)}\end{matrix}}} & (43)\end{matrix}$

FIG. 32 is a graph showing the relationship between the correctionfactor α₁ and the amount of change in the weight of the front wheels 150relative to its reference value (incidentally, the amount of change hasa positive value if the weight is more than the reference value, and theamount of change has a negative value if the weight is less than thereference value). For example, the optimum correction factor α₁ inaccordance with the amount of change in the weight of the front wheels150 is derived in advance on the basis of an empirical rule, as shown inFIG. 32. Then, the resonance compensators 27, 271 and 272 calculate thecorrection factor α₁ by substituting the amount of change in the weightof the front wheels 150 into a map showing the correspondence betweenthe amount of change in the weight of the front wheels 150 and thecorrection factor α₁, or a relational expression of the amount of changein the weight of the front wheels 150 and the correction factor α₁,which has previously been created and stored in the ROM, and uses thecalculated correction factor α₁ in the transfer function.

Incidentally, if the weight of the front wheels 150 is more than thereference value, the resonance frequency of the control system,inclusive of also the front wheels 150, with the steering apparatus 100,200 and 300 mounted on the vehicle, is lower than ω_(n) (where n=1, 2,3), or if the weight of the front wheels 150 is less than the referencevalue, the resonance frequency is higher than ω_(n). As shown in FIG.32, therefore, the correction factor α₁ is 1 if the weight of the frontwheels 150 is equal to the reference value, and the correction factor α₁decreases to 0.8 as the weight increases, and the correction factor α₁is set to 0.8 if the weight is more than a given value. Also,preferably, the correction factor α₁ increases to 1.2 as the weight ofthe front wheels 150 becomes lower than the reference value, and thecorrection factor α₁ is set to 1.2 if the weight is less than the givenvalue.

Also, it is preferable that the correction factor α₁ vary according tothe vehicle speed. FIG. 33 is a graph showing the relationship betweenthe correction factor α₁ and the vehicle speed. For example, the optimumcorrection factor α₁ in accordance with the vehicle speed is derived inadvance on the basis of an empirical rule, as shown in FIG. 33. Then,the resonance compensators 27, 271 and 272 calculate the correctionfactor α₁ by substituting the vehicle speed signal v into a map showingthe correspondence between the vehicle speed signal v and the correctionfactor α₁, or a relational expression of the vehicle speed signal v andthe correction factor α₁, which has previously been created and storedin the ROM, and uses the calculated correction factor α₁ in the transferfunction.

Incidentally, it is conceivable that the resonance frequency decreasesas the vehicle speed increases, and thus, as shown in FIG. 33,preferably, the correction factor α₁ is 1 when the vehicle speed iszero, the correction factor α₁ decreases to 0.8 as the vehicle speedincreases, and the correction factor α₁ is 0.8 when the vehicle speed ismore than a given speed.

Also, it is preferable that the correction factor α₁ vary according tothe angle of rotation (or a steering angle) of the steering wheel 101.FIG. 34 is a graph showing the relationship between the correctionfactor α₁ and the absolute value of the steering angle. For example, theoptimum correction factor α₁ in accordance with the absolute value ofthe steering angle is derived in advance on the basis of an empiricalrule, as shown in FIG. 34. Then, the resonance compensators 27, 271 and272 calculate the correction factor α₁ by substituting the detectedsteering angle into a map showing the correspondence between theabsolute value of the steering angle and the correction factor α₁, or arelational expression of the absolute value of the steering angle andthe correction factor α₁, which has previously been created and storedin the ROM, and uses the calculated correction factor α₁ in the transferfunction.

Incidentally, it is conceivable that the resonance frequency decreasesas the absolute value of the steering angle increases, and thus, asshown in FIG. 34, preferably, the correction factor α₁ is 1 when theabsolute value of the steering angle is zero, the correction factor α₁decreases to 0.8 as the absolute value of the steering angle increases,and the correction factor α₁ is 0.8 when the absolute value of thesteering angle is more than a given value.

Then, the resonance angular frequency corrected according to thecircumstances, as mentioned above, is used in the transfer functions ofthe resonance compensators 27, 271 and 272 thereby to permit achievingan improvement in the stability with higher accuracy and also animprovement in the response.

Also, it is preferable that a transfer function corrected bymultiplying, by β₁, the transfer functions H_(n)(s) (where n=1, 2, 3) ofthe resonance compensators 27, 271 and 272 according to theabove-mentioned sixth to eighth exemplary embodiments should be used asthe transfer function, where β₁ represents the correction factor.Specifically, the transfer function is corrected toH_(ng)(s)=β₁×H_(n)(s) (where n=1, 2, 3), which is then used.

FIG. 35 is a graph showing the relationship between the correctionfactor β₁ and the amount of change in the weight of the front wheels 150relative to its reference value (incidentally, the amount of change hasa positive value if the weight is more than the reference value, or theamount of change has a negative value if the weight is less than thereference value). For example, the optimum correction factor β₁ for theamount of change in the weight of the front wheels 150 is derived inadvance on the basis of an empirical rule, as shown in FIG. 35. Then,the resonance compensators 27, 271 and 272 calculate the correctionfactor β₁ by substituting the amount of change in the weight of thefront wheels 150 into a map showing the correspondence between theamount of change in the weight of the front wheels 150 and thecorrection factor β₁, or a relational expression of the amount of changein the weight of the front wheels 150 and the correction factor β₁,which has previously been created and stored in the ROM, and uses thecalculated correction factor β₁ in the transfer function.

Incidentally, if the weight of the front wheels 150 is more than thereference value, vibration, with the steering apparatus 100, 200 and 300mounted on the vehicle, increases, or if the weight of the front wheels150 is less than the reference value, the vibration decreases. As shownin FIG. 35, therefore, the correction factor β₁ is 1 if the weight ofthe front wheels 150 is equal to the reference value, the correctionfactor β₁ increases to 1.2 as the weight increases, and the correctionfactor β₁ is set to 1.2 if the weight is more than a given value. Also,preferably, the correction factor β₁ decreases to 0.8 as the weight ofthe front wheels 150 becomes lower than the reference value, and thecorrection factor β₁ is set to 0.8 if the weight is less than the givenvalue.

Also, it is preferable that the correction factor β₁ vary according tothe vehicle speed. FIG. 36 is a graph showing the relationship betweenthe correction factor β₁ and the vehicle speed. For example, the optimumcorrection factor β₁ in accordance with the vehicle speed is derived inadvance on the basis of an empirical rule, as shown in FIG. 36. Then,the resonance compensators 27, 271 and 272 calculate the correctionfactor β₁ by substituting the vehicle speed signal v into a map showingthe correspondence between the vehicle speed signal v and the correctionfactor β₁, or a relational expression of the vehicle speed signal v andthe correction factor β₁, which has previously been created and storedin the ROM, and uses the calculated correction factor β₁ in the transferfunction.

Incidentally, the vibration should be suppressed as the vehicle speeddecreases, and thus, as shown in FIG. 36, preferably, the correctionfactor β₁ is 1.2 when the vehicle speed is zero, and the correctionfactor β₁ decreases to 1 as the vehicle speed increases, and thecorrection factor β₁ is set to 1 when the vehicle speed is more than agiven speed.

Also, it is preferable that the correction factor β₁ vary according tothe angle of rotation (or the steering angle) of the steering wheel 101.FIG. 37 is a graph showing the relationship between the correctionfactor β₁ and the absolute value of the steering angle. For example, theoptimum correction factor β₁ in accordance with the absolute value ofthe steering angle is derived in advance on the basis of an empiricalrule, as shown in FIG. 37. Then, the resonance compensators 27, 271 and272 calculate the correction factor β₁ by substituting the detectedsteering angle into a map showing the correspondence between theabsolute value of the steering angle and the correction factor β₁, or arelational expression of the absolute value of the steering angle andthe correction factor β₁, which has previously been created and storedin the ROM, and uses the calculated correction factor β₁ in the transferfunction.

Incidentally, the vibration should be suppressed as the absolute valueof the steering angle decreases, and thus, as shown in FIG. 37,preferably, the correction factor β₁ is 1.2 when the absolute value ofthe steering angle is zero, and the correction factor β₁ decreases to 1as the absolute value of the steering angle increases, and thecorrection factor β₁ is set to 1 when the absolute value of the steeringangle is more than a given value.

Also, it is preferable that both the above-mentioned correction factorsα₁ and β₁ be used. Specifically, preferably, in the transfer functions“H_(n)(s)=a_(n)·((2πf_(c1))·(2πf_(c2)))/((s+2πf_(c1))·(s+2πf_(c2)))(where n=1, 2, 3)” of the resonance compensators 27, 271 and 272according to the above-mentioned sixth to eighth exemplary embodiments,a_(n) is multiplied by β₁, and also, the operator “s” for the Laplacetransform in a_(n) is replaced by “α₁×s,” and the corrected transferfunction is used as the transfer function of the resonance compensators27, 271 and 272.

Also in such an instance, the resonance compensators 27, 271 and 272calculate the correction factors α₁ and β₁ by substituting the amount ofchange in the weight of the front wheels 150 into a map showing thecorrespondence between the amount of change in the weight of the frontwheels 150 and the correction factors α₁ and β₁, or a relationalexpression of the amount of change in the weight of the front wheels 150and the correction factors α₁ and β₁, which has previously been createdand stored in the ROM, and uses the calculated correction factors α₁ andβ₁ in the transfer function. Incidentally, it is preferable that thecorrection factors α₁ and β₁ have the relationships shown in FIGS. 32and 35, respectively.

Also, the resonance compensators 27, 271 and 272 calculate thecorrection factors α₁ and β₁ by substituting the vehicle speed signal vinto a map showing the correspondence between the vehicle speed signal vand the correction factors α₁ and β₁, or a relational expression of thevehicle speed signal v and the correction factors α₁ and β₁, which haspreviously been created and stored in the ROM, and uses the calculatedcorrection factors α₁ and β₁ in the transfer function. Incidentally, itis preferable that the correction factors α₁ and β₁ have therelationships shown in FIGS. 33 and 36, respectively.

Also, the resonance compensators 27, 271 and 272 calculate thecorrection factors α₁ and β₁ by substituting the detected steering angleinto a map showing the correspondence between the absolute value of thesteering angle and the correction factors α₁ and β₁, or a relationalexpression of the absolute value of the steering angle and thecorrection factors α₁ and β₁, which has previously been created andstored in the ROM, and uses the calculated correction factors α₁ and β₁in the transfer function. Incidentally, it is preferable that thecorrection factors α₁ and β₁ have the relationships shown in FIGS. 34and 37, respectively.

Incidentally, in the above-mentioned sixth, seventh and eighth exemplaryembodiments, the resonance compensators 27, 271 and 272 have beendescribed as being configured to eliminate the resonance frequencycomponents of the torque signal Ts obtained through the phasecompensation by the phase compensator 26, and to output the torquesignal Tp obtained through the elimination of the resonance frequencycomponents; however, it is to be understood that the present inventionis not limited to such configurations.

For instance, the resonance compensators 27, 271 and 272 may beconfigured to eliminate the resonance frequency components of the torquesignal Td as the output value from the torque sensor 109 and to outputthe torque signal Tp, and the phase compensator 26 may be configured toperform the filtering processing for the phase compensation on thetorque signal Tp obtained through the elimination of the resonancefrequency components and output the torque signal Ts. In this instance,the base current calculator 21 calculates the base current on the basisof the torque signal Ts as the output value from the phase compensator26, and the vehicle speed signal v from the vehicle speed sensor 170.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theexemplary embodiments were chosen and described in order to best explainthe principles of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. An electric power steering apparatus, comprising: a steering torquedetector that detects steering torque of a steering wheel; an electricmotor that applies steering assist force to the steering wheel; acurrent detector that detects an actual current actually supplied to theelectric motor; a target current setting unit that sets a target currentto be supplied to the electric motor, on the basis of the steeringtorque detected by the steering torque detector; a feedback controllerthat performs feedback control so that the target current set by thetarget current setting unit and the actual current detected by thecurrent detector coincide with each other; and a feedforward controllerthat performs feedforward control for increasing the actual currentdetected by the current detector if the target current increases, thefeedforward controller including a frequency compensator that provides asmaller amount of increase in the actual current as a frequency of avariation in the target current is lower.
 2. The electric power steeringapparatus according to claim 1, wherein the feedforward controllerfurther includes a weighting processor that increases or reduces anamount of increase in the actual current by performing multiplication bya weighting factor set in advance.
 3. The electric power steeringapparatus according to claim 2, wherein the feedforward controllerfurther includes a weighting factor setting unit that sets the weightingfactor in accordance with at least any one of a vehicle speed of avehicle mounting the electric power steering apparatus and an amount ofchange in the steering torque detected by the steering torque detector.4. The electric power steering apparatus according to claim 3, whereinthe weighting factor setting unit sets the weighting factor larger asthe vehicle speed is lower.
 5. The electric power steering apparatusaccording to claim 3, wherein the weighting factor setting unit sets theweighting factor larger as the amount of change in the steering torqueis smaller.
 6. The electric power steering apparatus according to claim1, wherein the frequency compensator is a band-pass filter thattransmits a predetermined frequency alone.
 7. A control method of anelectric power steering apparatus comprising: detecting steering torqueof a steering wheel; detecting an actual current actually supplied to anelectric motor that applies steering assist force to the steering wheel;setting a target current to be supplied to the electric motor, on thebasis of the detected steering torque; performing feedback control sothat the target current and the actual current coincide with each other;performing feedforward control for increasing the actual current if thetarget current increases; providing a smaller amount of increase in theactual current as a frequency of a variation in the target current islower when the feedforward control is performed.
 8. A computer readablemedium storing a program, the program comprising the steps of: detectingsteering torque of a steering wheel; detecting an actual currentactually supplied to an electric motor that applies steering assistforce to the steering wheel; setting a target current to be supplied tothe electric motor, on the basis of the detected steering torque;performing feedback control so that the target current and the actualcurrent coincide with each other; performing feedforward control forincreasing the actual current if the target current increases; andproviding a smaller amount of increase in the actual current as afrequency of a variation in the target current is lower.
 9. An electricpower steering apparatus, comprising: a steering torque detector thatdetects steering torque of a steering wheel; an electric motor thatapplies steering assist force to the steering wheel; a current detectorthat detects an actual current actually supplied to the electric motor;a target current setting unit that sets a target current to be suppliedto the electric motor, on the basis of the steering torque detected bythe steering torque detector; a proportional controller that performs aproportional action of multiplying, by a proportional gain, a valuecorresponding to a current deviation between the target current set bythe target current setting unit and the actual current detected by thecurrent detector; an integral controller that performs an integralaction of multiplying, by an integral gain, an integral value obtainedby integrating the value corresponding to the current deviation; and anaddition unit that adds together an output value from the proportionalcontroller and an output value from the integral controller and outputsa command value to the electric motor, at least any one of theproportional controller and the integral controller including acorrection unit that performs multiplication by a correction factorthereby to enhance an effect of a corresponding one of the proportionalaction and the integral action, the electric power steering apparatusfurther comprising an adjusting unit that performs an adjustment so thata denominator of a transfer function remains constant regardless of avalue of the correction factor, when the target current set by thetarget current setting unit is an input, and the actual current actuallysupplied to the electric motor is an output.
 10. The electric powersteering apparatus according to claim 9, wherein the proportionalcontroller includes a correction unit that performs multiplication bythe correction factor thereby to enhance the effect of the proportionalaction, the adjusting unit multiplies, by a factor depending on thecorrection factor, the actual current detected by the current detector,and outputs a multiplication result, and the addition unit further addsan output from the adjusting unit, and outputs the command value to theelectric motor.
 11. The electric power steering apparatus according toclaim 10, wherein the factor depending on the correction factor is avalue obtained by multiplying, by the proportional gain, a valueobtained by subtracting 1 from the correction factor.
 12. The electricpower steering apparatus according to claim 9, wherein the integralcontroller includes a correction unit that performs multiplication bythe correction factor thereby to enhance the effect of the integralaction, the adjusting unit multiplies, by a factor depending on thecorrection factor, an integral value obtained by integrating a value ofthe actual current detected by the current detector, and outputs themultiplication result, and the addition unit further adds an output fromthe adjusting unit, and outputs the command value to the electric motor.13. The electric power steering apparatus according to claim 12, whereinthe factor depending on the correction factor is a value obtained bymultiplying, by the integral gain, a value obtained by subtracting 1from the correction factor.
 14. The electric power steering apparatusaccording to claim 9, wherein the correction factor varies in accordancewith at least any one of a vehicle speed of a vehicle mounting theelectric power steering apparatus and an amount of change in thesteering torque detected by the steering torque detector.
 15. Anelectric power steering apparatus, comprising: a steering torquedetector that detects steering torque of a steering wheel; an electricmotor that applies steering assist force to the steering wheel; acurrent detector that detects an actual current actually supplied to theelectric motor; a target current setting unit that sets a target currentto be supplied to the electric motor, on the basis of the steeringtorque detected by the steering torque detector; a first motor drivecontroller including: a first proportional controller that performs aproportional action of multiplying, by a proportional gain, a valuecorresponding to a current deviation between the target current set bythe target current setting unit and the actual current detected by thecurrent detector, and also that multiplies the multiplication result bya correction factor thereby to enhance an effect of the proportionalaction; a first integral controller that performs an integral action ofmultiplying, by an integral gain, an integral value obtained byintegrating the value corresponding to the current deviation; and afirst multiplication unit that multiplies, by a factor depending on thecorrection factor, the actual current detected by the current detector,the first motor drive controller adding together an output value fromthe first proportional controller, an output value from the firstintegral controller, and an output value from the first multiplicationunit, and outputting a command value to the electric motor; a secondmotor drive controller including: a second proportional controller thatperforms a proportional action of multiplying, by the proportional gain,the value corresponding to the current deviation; a second integralcontroller that performs an integral action of multiplying, by theintegral gain, the integral value obtained by integrating the valuecorresponding to the current deviation, and also that multiplies themultiplication result by a correction factor thereby to enhance aneffect of the integral action; and a second multiplication unit thatmultiplies, by a factor depending on the correction factor, an integralvalue obtained by integrating a value of the actual current detected bythe current detector, the second motor drive controller adding togetheran output value from the second proportional controller, an output valuefrom the second integral controller, and an output value from the secondmultiplication unit, and outputting a command value to the electricmotor; and a switching unit that performs switching between the firstmotor drive controller and the second motor drive controller to outputthe command value to the electric motor.
 16. The electric power steeringapparatus according to claim 15, wherein the switching unit performs theswitching in accordance with at least any one of a vehicle speed of avehicle mounting the electric power steering apparatus and the steeringtorque detected by the steering torque detector.
 17. An electric powersteering apparatus, comprising: a first rotary shaft connected to asteering wheel; a rack shaft that effects turning of a wheel to beturned, by a rectilinear motion; a second rotary shaft that effects therectilinear motion of the rack shaft; a torsion bar that provides aconnection between the first rotary shaft and the second rotary shaftand is twisted by operation of the steering wheel; an electric motorthat applies assist force for the operation of the steering wheel; asteering torque detector that detects steering torque of the steeringwheel; and a target current setting unit that sets a target current tobe supplied to the electric motor, on the basis of the steering torquedetected by the steering torque detector, the target current settingunit including a resonance compensator that is provided on an outputside of the steering torque detector and that suppresses a resonancefrequency component of a control system including the torsion bar as aspring element, and the electric motor, the second rotary shaft and therack shaft as inertial elements, and the target current setting unitsetting the target current in accordance with the steering torquesubjected to the suppression of the resonance frequency component by theresonance compensator.
 18. The electric power steering apparatusaccording to claim 17, wherein the resonance compensator has a filteringfunction and a low-pass filtering function, the filtering functionhaving an antiresonant element of the control system.
 19. The electricpower steering apparatus according to claim 17, wherein a numerator of atransfer function of the resonance compensator has the same element asthat of a denominator of a transfer function of the control system. 20.The electric power steering apparatus according to claim 19, wherein adenominator of the transfer function of the resonance compensator has adegree not less than a degree of the numerator.
 21. A control method ofan electric power steering apparatus including a first rotary shaftconnected to a steering wheel; a rack shaft that effects turning of awheel to be turned, by a rectilinear motion; a second rotary shaft thateffects the rectilinear motion of the rack shaft; a torsion bar thatprovides a connection between the first rotary shaft and the secondrotary shaft and is twisted by operation of the steering wheel; anelectric motor that applies assist force for the operation of thesteering wheel, the control method thereof comprising: detectingsteering torque of the steering wheel; suppressing a resonance frequencycomponent of a control system including the torsion bar as a springelement, and the electric motor, the second rotary shaft and the rackshaft as inertial elements; and setting a target current to be suppliedto the electric motor in accordance with the steering torque subjectedto the suppression of the resonance frequency component.
 22. The controlmethod of the electric power steering apparatus according to claim 21,wherein a filtering function and a low-pass filtering function are usedat the suppression of the resonance frequency component of the controlsystem, the filtering function having an antiresonant element of thecontrol system.